In comparison to the crystalline forms, amorphous drugs exhibit higher surface free energy, higher apparent solubility and faster dissolution rate. However, since the amorphous state is thermodynamically unstable, amorphous solids tend to transform into their stable crystalline forms, resulting in the decreased dissolution and hence low bioavailability. This paper reviews the fundamental introduction of amorphous drugs, the crystallization theory, the influence factors on the physical stability and strategies for their crystallization inhibition, in order to provide a theoretical guidance for the rational design of amorphous drug products and efficient development of amorphous formulations. All of these could contribute to more applications of amorphous drugs in overcoming the poor water solubility issues.
Contents:
1 Introduction
2 Overview of amorphous drugs
3 Crystallization of amorphous drugs
3.1 Crystallization behaviors of amorphous drugs
3.2 Crystallization tendency of amorphous drugs
3.3 Crystallization theory
4 Influence factors on crystallization of amorphous drugs
4.1 Thermodynamic factors
4.2 Kinetic factors
4.3 Intermolecular interactions
4.4 Water vapor sorption
4.5 Formulation preparation
5 Strategies to inhibit crystallization of amorphous drugs
With the development of high-throughput screening technology, genomics, combinatorial chemistry and computational methods, the number of new chemical entities has increased dramatically in the past decade[1]. In recent years, 75% of new drug candidates face the problem of poor water solubility, which brings great challenges to the development of pharmaceutical formulations[2]. The traditional methods to improve the solubility and dissolution rate of drugs include salt formation, solubilization and complexation, but the application of these traditional methods is limited because of the arduous process of screening high soluble salts and the need for a large number of solubilizers and complexing agents[3].
Amorphous technology is an effective method developed rapidly in recent years to improve the solubility and dissolution of poorly soluble drugs, which transforms crystalline drugs with ordered intermolecular stacking into amorphous drugs with disordered intermolecular arrangement[4]. Amorphous drugs have higher surface free energy and generally exhibit higher apparent solubility, faster dissolution rate, and ultimately higher bioavailability in vivo[5,6]. Amorphous forms of poorly soluble drugs have been widely used in marketed drugs, such as cefuroxime axetil, nelfinavir mesylate, quinapril hydrochloride, rosuvastatin calcium and zafirlukast[7,8].
Despite the potential advantages of amorphous drugs, their application in oral solid formulations is still limited, mainly due to the instability of the amorphous form[9,10]. Compared with the crystalline state, the thermodynamic instability and higher molecular mobility of the amorphous state lead to the tendency of spontaneous crystal transformation[11]. Amorphous drug may undergo crystallographic transformation during manufacture, storage, or dissolution, thereby losing solubility and dissolution benefits, and many amorphous solid dispersions have been recalled by the FDA due to this physical instability and the consequent decrease in dissolution[12]. Therefore, understanding the influencing factors and inhibition methods of drug amorphous transformation is not only of great scientific significance, but also of practical application value.
In this paper, the essence of amorphous drug, the theory of crystallization, the influencing factors of crystal transformation and the strategies of inhibiting crystal transformation were introduced, which provided theoretical guidance and application reference for improving the stability of amorphous drug in preparation development.
2 Overview of Amorphous Forms of Drugs
In drug development, different solid forms of the same drug are obtained due to different solidification processes, and different solid forms usually show different properties due to different molecular arrangements, such as solubility, bioavailability, physical and chemical stability, and mechanical properties[13]. Different from the long-range order and periodic arrangement of the crystalline state, the amorphous state presents long-range disorder and short-range order, which makes the amorphous state have higher enthalpy, entropy, free energy and volume than the crystalline state, thus having higher apparent solubility[14]. Table 1 shows the difference in physicochemical properties between the amorphous and crystalline forms of the drug.
Fig. 1 is an enthalpy-temperature diagram for an amorphous system. When the undegraded molten drug is slowly cooled, the molecules have enough time to move from the current position to the thermodynamic stable point on the lattice and form a crystal structure in an orderly arrangement[15]. However, when the cooling rate is fast enough, the drug will not crystallize but enter the supercooled liquid state. Although the supercooled liquid state is a non-equilibrium state relative to the crystalline state, it is actually an equilibrium state from the point of view that the structure changes with temperature[15]. In other words, in essence, cooling causes instantaneous changes in the consistency of structural and thermodynamic properties, so the supercooled liquid is said to be in "structural equilibrium"[15]. When the drug is further cooled below the glass transition temperature (glass transition temperature,Tg), the viscosity increases, and the molecular motion is so slow that the rearrangement of amorphous internal structure can not catch up with the temperature change, so that the system is no longer in a state of structural equilibrium, which is called a glass state[15]. The Tg is a parameter to distinguish the amorphous state from the crystalline state. Whether the glass transition is a thermodynamic phenomenon or a kinetic phenomenon has been a controversial issue for a long time. From the thermodynamic point of view, this is an underlying thermodynamic transition that is kinetically controlled and is associated with sudden changes in some thermodynamic properties. From the kinetic point of view, the change of structure is the result of the kinetic transition in the amorphous relaxation process, and has nothing to do with the sudden change of thermodynamic properties[16].
3 Amorphous transformation of drug
3.1 Crystal Transformation Behavior of Amorphous Drug
Because the amorphous state is in a high energy state and belongs to a thermodynamically unstable system, it is easy to transform to a stable crystal form due to the influence of pressure, temperature, humidity and other environments in the process of preparation, storage and dissolution. For example, when amorphous ranitidine hydrochloride is prepared by ball milling, the drug is first transformed from crystalline to amorphous with the progress of milling, and then the amorphous is gradually transformed into crystalline under the influence of temperature and pressure[17].
It is very common for amorphous to undergo crystal transformation during storage. For example, after the amorphous docetaxel prepared by rotary evaporation was placed at 25 ℃/60% RH for 7 days, the powder X-ray diffraction results showed that it had undergone obvious crystal transformation and poor physical stability[18]. Grzybowsk et al. placed the amorphous celecoxib prepared by melt-quenching method under the condition of 20 ℃/10% RH, and the powder X-ray diffraction results showed that it underwent crystal transformation within 3 days[19].
Amorphous drug will rapidly dissolve to a peak during dissolution, and then due to solution-mediated crystal transformation, the dissolution rate decreases and the advantage of high solubility is lost. Hancock et al. determined the dissolution behavior of excessive amorphous and crystalline indomethacin in water at different temperatures, and found that the concentration of amorphous indomethacin reached the maximum at about 10 min, which was 3 to 5 times that of the crystalline form, and then the crystal transformation occurred, and the concentration decreased to the level of crystalline solubility[20].
The rapid dissolution of amorphous drugs often leads to the formation of supersaturated solutions, which are in a thermodynamic metastable state and prone to precipitation[21]. As shown in fig. 2, that rapid dissolution of the amorphous drug in the dissolution medium result in a supersaturated solution, followed by a rapid decrease in the concentration of the rapidly crystallize amorphous drug (E. G., docetaxel) to the crystalline solubility level (Ceq), a process similar to the stretching and recovery of a "spring"[18]; Slow crystalline amorphous forms (such as tadalafil) follow the Ostwald rule for slow nucleation and crystal growth, with a slow decrease in solubility and a period of supersaturation, similar to a "parachute"[22][23].
3.2 Amorphous crystallization tendency of drug
According to the crystallization behavior of organic molecules during the heating – cooling – heating DSC cycle, the crystallization tendency of drug amorphous can be classified (Figure 3). Baird et al. Studied the crystallization behavior of 51 kinds of organic small molecules in the process of heating (10 ℃/min) -cooling (20 ℃/min) -heating (10 ℃/minute) by DSC, and found that some compounds were easy to crystallize in the process of cooling, such as haloperidol and tolbutamide, which showed a high crystallization tendency and were called Class I compounds[25]. Class II compounds, such as paracetamol and cinnarizine, do not crystallize in the cooling process, but crystallize in the subsequent heating process. Class III compounds, such as aceclofenac and clotrimazole, do not crystallize during the heating – cooling – heating cycle, and have a low tendency to crystallize. The physical stability of type I and type II compounds is mainly affected by temperature, while that of type III compounds is mainly affected by molecular interaction. In addition, it was found that the heating/cooling rate in the DSC cycle would affect the classification of drug crystallization tendency. For example, nilutamide did not crystallize in the process of cooling (20 ℃/min) and heating again (10 ℃/min), which belonged to class III compounds, but crystallized in the process of heating again at a slow rate (2 ℃/minute), which belonged to class II compounds.
Glass forming ability (GFA) describes the ease with which a compound forms an amorphous form during the preparation process (such as melt-quenching, solvent evaporation, etc.), and is characterized by brittleness (or steepness coefficient, m). Glass stability (GS) is used to describe the ability of amorphous solids to resist crystallization. Both GFA and GS can reflect the inherent tendency of amorphous crystallization, and there is a close relationship between them. In general, drugs with low GFA crystallize faster and exhibit lower GS[25,27].
Brittleness is the degree of sensitivity of the amorphous structure to changes in temperature and is used to assess the dynamic behavior of the amorphous[28]. It is found that the relaxation times of two compounds with similar Tg but different brittleness can differ by several orders of magnitude[29]. Brittleness is defined as the change in relaxation time with temperature at Tg (Eq. 1), which can be approximated using the apparent activation enthalpy ΔH* as Eq. 2:[30][31]
Where R is the gas constant; τ is the relaxation time; The ΔH* can be calculated by measuring the Tg at different heating rates (Q) by DSC, as shown in the following formula:
The brittleness of the amorphous can be calculated by the dependence of the Tg on the heating rate. According to the brittleness, the amorphous can be divided into rigid and brittle. Rigid amorphous is a good glass former, which can resist structural rearrangement and maintain local structural order during the transition from supercooled liquid state to glass state[32]. The rigid behavior is a predictable Arrhenius-type behavior with an activation energy that is independent of temperature[16]. Brittle amorphous is a weak glass former, and its structure will change greatly with the change of temperature. The brittle behavior deviates from the Arrhenius law obviously, and the activation energy is related to temperature. Brittle amorphous is more prone to crystal transformation during storage, while most of the drug amorphous exhibits brittle behavior[33].
3.3 Amorphous crystal theory
Amorphous crystal transformation is the combined result of nucleation and crystal growth, which have different temperature dependence and jointly control the amorphous crystal transformation behavior of drugs.
As shown in Fig. 4, the free energy difference (ΔGv) between the amorphous state and the crystalline state of the drug is the main thermodynamic driving force for crystallization, and the higher the degree of supercooling (i.e., the lower the temperature), the stronger the thermodynamic driving force[34]. Nucleation is thermodynamically disadvantageous because of the need to create new surfaces, i.e., an increase in free energy (ΔGa). Thus, the driving force for nucleation is the net difference between ΔGv and ΔGa. In addition, there is a kinetic factor that depends on the diffusion of the system, and the slower the diffusion, the slower the nucleation rate.
Nucleation kinetics can be described by the Classical Nucleation Theory (CNT) as follows:[11]
Where J is the steady-state nucleation rate; A is the frequency factor, independent of temperature; .gamma. Is the interfacial tension between the crystalline surface and the amorphous; K is Boltzmann's constant; The T* is the solid-liquid equilibrium temperature; The ΔHf is the latent heat of fusion; Tr=T/T*;ΔTr=ΔT/T*=1-Tr, where T is the temperature and ΔT(=T*-T) is the degree of supercooling; ΔG 'is the kinetic energy barrier to nucleation, i.e., the activation free energy of molecular motion (diffusion), which is usually expressed as a function of molecular motion distance and viscosity[35]. According to Formula 4, it is obvious that the thermodynamic driving force for nucleation is opposite to the sign of the kinetic factor. When the temperature is low, the supercooling degree of the system is high, the molecular fluidity is low, and the viscosity of the system is high. The thermodynamic driving force is conducive to nucleation. When the temperature is high, the molecular mobility is high, and the kinetic factors are beneficial to the nucleation. Therefore, nucleation occurs as a result of the balanced action of two factors.
CNT assumes that this spontaneous nucleation occurs at a constant rate, but studies have found that the actual nucleation process begins at a faster rate, slows down over time, and eventually proceeds at a steady-state rate (Figure 5)[36]. Zeldovich proposed a non-steady-state nucleation model (Equation 5):[37]
Where, I(jc,t) is the nucleation rate for the formation of the critical size jc, which is related to time; The τ# is the time to form the critical dimension jc and reach the steady-state distribution; The Iss is the steady-state nucleation rate, i.e., the nucleation rate J in the classical nucleation theory (Equation 4).
After the crystal nucleus is formed, it continues to grow to form a crystal, which is the crystal growth stage. The mathematical model describing the crystal growth rate U is as follows:[38]
Where C is a constant; T is the temperature; η is the viscosity of the system, reflecting the molecular fluidity; The ΔGv is the free energy difference between the amorphous state and the crystalline state; W is a constant related to the growth mechanism; K is Boltzmann's constant. Equations 4 and 6 show that nucleation and crystal growth have different temperature dependencies. Fig. 6 is a classic graph depicting the nucleation rate and crystal growth rate as a function of temperature.
As the temperature approaches the Tg, the nucleation rate approaches zero. When the temperature is close to the melting point Tm, the crystal growth rate is close to zero. The nucleation rate is higher than the nucleation growth rate at higher supercooling state. In the lower supercooled state, the nucleation rate is higher than the nucleation rate. The shaded area in fig. 6 indicates a process in which nucleation and crystal nucleus growth proceed simultaneously, that is, a crystal transformation process occurs between the Tg and the Tm. Nucleation and slow growth of the nuclei sometimes occur below the Tg, which is caused by relaxation processes[39]; At temperatures just below the melting point, some substances can crystallize despite the small thermodynamic driving force[11].
The entire crystallization process of nucleation and crystal growth can be described by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation (Equation 7), which can be used to calculate the crystallization rate constant:[40]
Where α (t) represents the fraction of the transformation at time t; The t0 is crystallization induction time; The K=kn is the apparent crystallization rate constant and is the average of the nucleation rate and the crystal growth rate, and K is the crystallization rate[41]; Avrami exponent n is related to the dimension of crystal growth and nucleation mechanism. The smaller the value of n is, the smaller the dimension of crystal growth is, the slower the nucleation rate is, and the crystallization mechanism is gradually changed from nucleation control to diffusion control[42].
4 Influencing factors of amorphous transformation of drugs
4.1 Thermodynamic factor
The Tg and molecular mobility of amorphous felodipine and nifedipine are similar.However, the stability of amorphous felodipine is better than that of amorphous nifedipine, which is attributed to the lower free energy difference between amorphous and crystalline felodipine, and the higher activation energy barrier for nucleation, so the thermodynamic driving force for crystallization is smaller[43]. The magnitude of the free energy difference depends on the melting heat and heat capacity of the amorphous and crystalline States, and the Hoffmann formula (Formula 8) can accurately predict the relationship between the change of the free energy difference between the amorphous and crystalline States and the temperature:
Wherein the ΔGv is the free energy difference between the amorphous state and the crystalline state; The ΔHf is the heat of fusion of the crystalline state; The Tm is the melting point; T is the temperature. According to the formula 8, when the melting heat of the sample is larger, the ΔGv is also larger, that is, the thermodynamic driving force of the crystal transformation is larger. Fig. 7 is a plot of the free energy difference predicted by Hoffmann's formula as a function of temperature for different heats of fusion (55 J/gm, 110 J/gm, 220 J/gm). It can be seen from fig. 7 that at the same temperature, the larger the heat of fusion is, the larger the free energy difference between the amorphous state and the crystalline state is, and the greater the driving force for nucleation is, so that crystallization is more likely to occur.
In addition, there are other thermodynamic factors that affect the transformation of amorphous crystals. Acetaminophen, sucrose, fenofibrate and ritonavir were used as model drugs.The results showed that the compounds with low configurational entropy required low molecular mobility to crystallize spontaneously, and the system with molecular conformation orientation suitable for crystallization was more likely to nucleate and undergo crystal transformation[44]. Carpentier et al. Found in another study of four amorphous forms of pentitol that the different crystallization behavior of the amorphous forms was attributed to the difference in interfacial energy related to molecular conformation[45].
4.2 Kinetic factor
The transformation of the amorphous state generally occurs in the glass transition and melting temperature range (fig. 6), but it can also be detected below the Tg, which is inseparable from the molecular motion at this temperature. Below the Tg, the amorphous solid is in a glassy state and undergoes structural relaxation toward equilibrium, in which the energy, enthalpy, and free volume all decrease (Figure 1). Diffusive structural relaxation (α-relaxation) and non-diffusive Johari-Goldstein (JG) relaxation (β-relaxation) are the main relaxation modes of amorphous[46,47]. α-relaxation is the main relaxation, which refers to the molecular motion in a larger unit and a larger range, reflects the mobility of the whole molecule, and does not follow the Arrhenius law. β-relaxation is a secondary relaxation, which usually originates from the high-speed and frequent motion of local molecules, which is not coupled or correlated with the motion of the whole molecule and follows the Arrhenius law[48]. The energy barrier for α-relaxation is very high because of the cooperative motion of the molecules. The β-relaxation is a local non-cooperative motion, so the energy barrier is very low and plays an important role at low temperatures.
In general, the relaxation associated with amorphous stability is mainly α-relaxation. It was found that the crystallization of amorphous griseofulvin and nifedipine was strongly correlated with α-relaxation, but not with β-relaxation, whether above Tg or below Tg[49]. Bhardwaj et al. studied the physical stability of amorphous itraconazole and amorphous trehalose above Tg and found that both were associated with α-relaxation[50][51]. However, when the temperature is very low, the molecular mobility of the amorphous bulk is very low, and the α-relaxation is negligible. At this time, the β-relaxation is the main kinetic factor affecting the stability of the amorphous[52]. For example, there is an extremely strong correlation between the physical instability of amorphous celecoxib below Tg and β-relaxation[53]. When the temperature is about Tg, α-relaxation is the main factor in the process of indomethacin amorphous nucleation. When the temperature is much lower than Tg, β-relaxation is the main factor affecting the nucleation[54].
Molecular mobility is often expressed in terms of the reciprocal of the relaxation time (τ), which can be calculated from the Adam-Gibbs-Vogel (AGV) equation as follows:[55]
Where τ is the average relaxation time; τ0 is a time constant of about 10-14s[15]; D is the strength coefficient[42,56]; The T0 is the temperature at which molecular mobility is zero[57]; T is the temperature of the amorphous system; Tf is the virtual temperature at which the thermodynamic properties (such as entropy and enthalpy) of the equilibrium glassy state are similar to those of the non-equilibrium glassy state at temperature T. For most of the freshly prepared amorphous, when T<Tg, the configurational entropy does not change significantly with temperature, Tf≈Tg; When the T>Tg,Tf=T,AGV equation can be reduced to the Vogel-Tammann-Fulcher (VTF) equation, i.e.
D and T0 in Equation 10 are the parameters of VTF equation, which can be obtained from the brittleness (m) (Equations 11 to 13), and there is an approximate relationship between D and T0, which is Tg/T0=1+D/39.1.
The relaxation time of the amorphous drug at different temperatures can be calculated according to Formulae 1 to 3 and Formulae 9 to 13. Below the Tg, the molecular motion is very slow and generally does not become the driving force for nucleation. In this case, the nucleation is essentially thermodynamic, and the molecular mobility only plays a role in promoting[34].
The crystal transformation of amorphous solids is controlled by both thermodynamic and kinetic factors, which can be expressed by the following formula:[57,58]
Where K represents the rate of crystallization of the amorphous form of the drug at temperature T; D (T) represents the temperature-dependent molecular motion at the nucleus-amorphous interface; F (T) represents the nucleation free energy, i.e., the thermodynamic driving force for nucleation. When D (T) is much larger than f (T), that is, the influence of molecular motion on the crystallization rate is much larger than that of thermodynamic factors, the following relationship holds:[30,59]
Where K and represent the crystallization rate at the temperature of T and Tg, respectively; The DT and the represent the diffusion coefficients of the amorphous substance at T and Tg, respectively; The viscosity of the system when η and are T and Tg; The is the relaxation time at Tg, about 100 s; ξ is the correlation exponent (0 ≤ 0 ≤ ξ ≤ 1), which is a decoupling constant used to compensate the relationship between the diffusion coefficient and the viscosity η of the system, indicating the degree of correlation between the molecular mobility and the crystallization rate. When ξ = 1, the crystallization rate depends entirely on the molecular mobility; When ξ < 1, the crystallization rate measured by experiment is less than that calculated by AGV equation, which indicates that the crystallization rate is partly related to the molecular fluidity[30].
Zhou et al. Found that the amorphous griseofulvin has ξ < 1, indicating that its crystallization rate is partially related to molecular fluidity, and this incomplete correlation may be the result of decoupling between molecular diffusion and viscosity[41]. Other studies have found that the ξ of amorphous phenobarbital is equal to 1, indicating that there is a very good correlation between the crystallization rate and the relaxation time, and the molecular fluidity has a significant impact on the stability of amorphous phenobarbital. Amorphous nifedipine is ξ < 1 below the Tg and ξ = 1 above the Tg[30].
4.3 Molecular Interaction
Intermolecular forces include van der Waals forces, hydrogen bonds and other non-covalent bonds. It is found that hydrogen bonds are closely related to the mobility of amorphous molecules[60]. The molecular motion needs to destroy the original intermolecular force and then form a new force when the amorphous drug changes its crystal form. Therefore, the stronger the intermolecular hydrogen bond of the amorphous drug is, the greater the influence on the relaxation time is, and then the molecular fluidity and crystallization tendency of the amorphous drug are affected.
Tang et al. compared the strength of hydrogen bonds in crystalline and amorphous forms of seven dihydropyridine drugs. The results showed that the hydrogen bonds in crystals of some drugs (such as nimodipine and nifedipine) were stronger than those in amorphous forms, while the hydrogen bonds in amorphous forms of other drugs (such as felodipine and eladidipine) were stronger than those in crystals[61][61]. Subsequent studies have found that the difficulty of amorphous transformation can be predicted by comparing the size of hydrogen bonds in crystal and amorphous. When the hydrogen bonds in the amorphous state are stronger than those in the crystalline state, some hydrogen bonds need to be destroyed for crystallization to occur, so the crystal transformation is slower. Marsac et al. compared the physical stability of amorphous nifedipine and felodipine, and found that the hydrogen bond of crystalline nifedipine was stronger than that of amorphous nifedipine, while the opposite was true for felodipine, with the hydrogen bond of amorphous being stronger than that of crystalline[43]. It was found that the stability results were consistent with expectations, and amorphous nifedipine was more likely to undergo crystal transformation.
4.4 Water adsorption
Compared with crystalline substances, amorphous substances have a higher affinity for water and can spontaneously absorb water from the surrounding environment. Water has a strong plasticizing effect, which can reduce the Tg of the system and increase the molecular fluidity, thus accelerating the occurrence of amorphous crystal transformation[29].
After a certain amount of water is absorbed by an amorphous drug, the change of the Tg obeys the Gordon-Taylor equation:[62]
Among,The Tgmix is the Tg;Tg1 after absorbing a certain amount of water, and the Tg2 is the Tg;w1 of water and drug amorphous.And w2 are the weight fractions of water and drug amorphous, respectively (w/w);ρ1 and ρ2 are the densities of water and drug amorphous, respectively; Tg (water) = 135 K[63].
Miller et al. Studied the effect of moisture on the amorphous raffinose dried by spray drying, and found that with the increase of water content in the amorphous system, the relaxation time decreased and the rate of crystal transformation increased[64]. Rumondor et al. Studied the effect of humidity on the growth rate of amorphous felodipine crystals and found that the growth rate of crystals increased approximately logarithmically with the increase of humidity in the range of 13% ~ 80% RH[65].
4.5 Prescription Technology
4.5.1 Amorphous preparation method
The preparation methods of amorphous drug include melt-quenching, grinding, solvent evaporation and desolvation, etc. Different preparation methods will introduce different thermal history and mechanical force to the amorphous sample, resulting in different physical stability of the amorphous sample.
Amorphous crystals prepared by grinding usually have poor stability, which may be due to the highly defective crystals produced by ball milling, and the residual crystal nuclei in amorphous crystals can cause nucleation and crystal growth, thus increasing the risk of crystal transformation during storage[66,67]. For example, the amorphous form of tri-O-methyl-β-cyclodextrin prepared by the grinding method is more susceptible to crystallization than that prepared by the melt-quenching method[68]. Crowley et al. Found that the crystallization behavior of amorphous indomethacin prepared by low temperature grinding method depends on its initial crystal form, which indicates that the amorphous after grinding has the "memory" of the long-range arrangement of the original crystal[69].
Surana et al. Prepared trehalose amorphous by different methods, and found that the preparation methods had little effect on the Tg and brittleness of the amorphous, but significantly affected the enthalpy relaxation, crystallization tendency and water absorption[70]. The physical stability of trehalose amorphous prepared by dehydration method was the worst, which may be due to the high temperature and long duration of dehydration method, resulting in high molecular mobility and nucleation density. Edueng et al. compared the effects of spray drying and melt-quenching on the amorphous physical stability of different drugs, and found that the amorphous prepared by spray drying had worse stability, which may be due to the relatively rapid evaporation provided by spray drying, resulting in hollow and porous particles, and the larger particle surface area made the amorphous easier to transform into crystals[68].
4.5.2 Formulation process of preparation
After the amorphous drug is prepared, it needs to go through many unit operations (granulation, tabletting, drying, coating, etc.) Together with appropriate excipients to obtain the final preparation product. The addition of excipients may promote or inhibit the amorphous transformation of drugs. For example, highly hygroscopic excipients (such as anhydrous lactose and microcrystalline cellulose) may promote the amorphous transformation of drugs, while polymeric excipients often inhibit the transformation. On the one hand, surfactants induce nucleation by reducing the viscosity of the amorphous, reducing the amorphous-crystalline interfacial energy, or introducing nuclei; on the other hand, surfactants may adsorb on the interface, thereby inhibiting crystal growth[11]. Ghebremeskel et al. Studied the combined effects of surfactants and polymers on the stability of amorphous solid dispersions, and found that surfactants produce plasticization, reduce the viscosity of the system, and promote amorphous crystal transformation[71]. However, choosing the appropriate polymer can produce anti-plasticizing effect, eliminate the plasticizing effect of surfactant, and finally obtain solid dispersion with good physical stability.
In the whole process of preparation production, the physical stability of amorphous drug needs to be paid attention to. For example, the amorphous drug will be affected by solvent, temperature or mechanical force during granulation, drying and tabletting. The addition of binder during the wet granulation process may lead to solution-mediated crystal transformation, and the subsequent drying process may also lead to a relaxation change in the amorphous structure and an increase in the rate of crystal transformation. The dry granulation process does not use a binder solution, thus avoiding the effect of moist heat on the amorphous transformation, but the mechanical force applied in this process may induce the amorphous transformation.
In particular, the physical stability of the amorphous form of the drug needs to be paid attention to in the process of tabletting, and some amorphous forms, such as celecoxib, etoricoxib and indomethacin, will undergo crystal transformation under less pressure[72][73][74]. Our research group found that the pressure of 225 MPa and above will lead to the amorphous transformation of repaglinide. Imamura et al. Found that even if the amorphous sucrose did not transform immediately after compression, the nucleation induction time would be shortened in the subsequent storage process, indicating that compression promoted the formation of critical nuclei, which may be due to the deformation of sucrose molecular conformation caused by compression and the increase of free energy of the system[75].
In the coating process, the choice of coating solution may also affect the physical stability of the amorphous. Coating can be divided into non-functional coating and functional coating. Non-functional coatings are solvent-based and require efficient air exchange to minimize the interaction between the coating core and the coating solvent during the coating process, resulting in less chance of crystal transformation of the drug in the tablet core when using this method. If necessary, the core may be barrier coated prior to film coating to prevent solid-liquid interactions during film coating. The coating layer for modifying the release of the product in a functional coating may contain a drug, which is generally obtained by spraying a drug solution or a drug suspension onto the surface of the coating core, during which process the amorphous form of the drug may undergo crystal transformation. The main reason is that when the drug is coated with a solution, the rapid removal of the solvent during the coating process may cause amorphous crystal transformation in the drug layer; When a drug suspension is used for coating, a solution-mediated phase transition may occur if a more stable phase is present in the aqueous medium[76]. When Lust et al. Coated microcrystalline cellulose neutral pellets (Cellet@) with a coating solution containing an amorphous solid dispersion of piroxicam -Soluplus@, they found that the amorphous form of piroxicam was converted to the monohydrate[77].
The above factors affecting the amorphous stability are often not a single factor at work, but the result of a combination of factors. Among the various factors, the free energy difference between the amorphous state and the crystalline state is the driving force for the crystal transformation of the amorphous sample, and the molecular fluidity plays a key role in the crystal transformation process of the amorphous material.The physical stability of the amorphous phase is affected by the intermolecular forces of the amorphous sample, the thermal or mechanical stress of the preparation process, the water adsorption, and the formulation process of the preparation.
5 Strategies for Inhibition of Amorphous Transformation of Drugs
5.1 Store at low temperature
Obviously, the molecular motion of amorphous drugs is slower when stored at low temperature, which is beneficial to physical stability. Fig. 8 shows the changes in the relaxation times of amorphous sucrose, indomethacin and polyvinylpyrrolidone (PVP) after storage at different temperatures below Tg for a certain period of time[78]. The relaxation time increases with the decrease of storage temperature.When the temperature reaches 50 K at Tg, the relaxation time is the longest and the relaxation process is the slowest, which can be ignored.
When the temperature is at the Tg-T≥50 K, the viscosity is infinite and the molecular motion reaches the "frozen" state. Therefore, in order to improve the stability of the amorphous drug during storage, it can be selected to be stored at 50 K below Tg. Understanding the change of relaxation time with storage temperature can provide a certain basis for the determination of storage conditions and shelf life of amorphous raw materials and preparations of drugs.
5.2 Preparing solid dispersion
It is an effective method to improve the stability of amorphous solid dispersion by dispersing drugs in polymer solid carrier in amorphous state. So far, there are many solid dispersion preparations on the market, such as etravirine-hydroxypropylmethylcellulose (HPMC), fenofibrate-polyethylene glycol (PEG), ivakato-hydroxypropylmethylcellulose acetate succinate (HPMCAS) and ritonavir-PVP VA64[8].
Amorphous solid dispersion also exhibit significantly higher apparent solubility and dissolution rate than crystalline drug due to that highly disordered arrangement of the drug molecules. Amorphous solid dispersion have higher physical stability than amorphous drug, mainly due to that follow reasons: (1) the carrier materials in solid dispersion tend to have higher Tg, and the anti-plasticizing effect of the carrier materials can improve the Tg of amorphous drugs, thereby slowing down the occurrence of crystal transformation; (2) from a thermodynamic point of view, when the drug is mixed with the polymer, the chemical potential energy of the amorphous form of the drug is reduced and the energy barrier to crystallization is increased (fig. 9), thereby increasing the stability of the amorphous form of the drug; (3) The ion-dipole moment or hydrogen bond formed between the drug and the polymer molecules reduces the amorphous molecular mobility of the drug, hinders the binding between the drug molecules, and thus inhibits the crystal transformation of the drug[79]. In 2019, Han et al. used machine learning technology to establish a prediction model for the physical stability of amorphous solid dispersions, which is beneficial to the rational design of solid dispersion prescriptions and the avoidance of time-consuming and labor-intensive trial-and-error experiments[80].
Although amorphous solid dispersion technology has become one of the most promising methods to improve the solubility and oral bioavailability of poorly soluble drugs, the preparations on the market have shown good efficacy.However, the development of amorphous solid dispersions is still restricted by many factors, such as the hygroscopicity of solid dispersions, the poor compatibility between some drugs and carrier materials, and the large amount of carrier materials leading to a large amount of dosage forms[81,82].
5.3 Prepared into co-amorphous
Pharmaceutical co-amorphous refers to the single-phase amorphous system formed by active pharmaceutical ingredients and other physiologically acceptable small molecular substances or small molecular pharmaceutical excipients through non-covalent bonds such as hydrogen bonds, ionic bonds or non-interaction forces.These co-amorphous systems have good stability and can improve the solubility and dissolution rate of poorly soluble drugs[83].
A ligand that generally has a high Tg can increase that Tg of the co-amorphous system, as compare to the amorphous form of the drug alone, thereby increasing its stability. There are often hydrogen bonds, ionic bonds and other interactions between the co-amorphous components, which inhibit the crystal transformation by reducing the free energy and molecular mobility of the system (Fig. 10). Our research group found that in the co-amorphous form of lurasidone hydrochloride and L-cysteine hydrochloride, hydrogen bonding interaction was formed between the two components, which significantly improved the physical stability of lurasidone hydrochloride[84]. There is no interaction in some co-amorphous compounds, but the uniform mixing of different components at the molecular level hinders the direct contact of the same drug molecule, thus inhibiting its crystal transformation. For example, our group found that there was no interaction between the two components in the co-amorphous form of lurasidone hydrochloride and repaglinide, and the good stability of the co-amorphous form was attributed to the uniform mixing of the two components at the molecular level and the uniform steric effect[85]. Knapik et al. prepared ezetimibe with a small amount of indapamide into co-amorphous form, which significantly improved the stability of ezetimibe, mainly due to the anti-plasticizing effect of indapamide[86].
Co-amorphous technology can effectively avoid the hygroscopic problem of polymer carriers in amorphous solid dispersions. In addition, the other component of co-amorphous drugs usually uses small molecular excipients or drug molecules with synergistic effect, and the dosage is significantly lower than that of high molecular polymers in solid dispersions. For example, the adult clinical dose of curcumin, a poorly soluble drug, is about 1 to 2 G/d, and its solid dispersion consists of curcumin and the polymer HPMCAS in a weight ratio of 1:9. Such a high polymer ratio results in a large total dosage form (10 to 20 G/d)[87]. However, the preparation of curcumin and piperine, a small molecule ligand, in a 1:1 molar ratio (approximately 1: 0.77 weight ratio) into a co-amorphous form greatly reduces the total dosage form[88].
In addition, our research group found that the co-amorphous lurasidone hydrochloride and saccharin not only improved the amorphous stability of lurasidone hydrochloride, but also solved the problem of pH-dependent dissolution[89]. While the co-amorphous lurasidone hydrochloride and puerarin improved the stability of the two drugs, the two drugs were dissolved simultaneously[90].
In general, the improvement of co-amorphous stability is the result of a combination of these mechanisms. For example, in most drug-amino acid coamorphs, both the increase in Tg and molecular interactions contribute to the inhibition of crystallographic transformation.
5.4 Optimize the prescription and technology
The preparation conditions of drug amorphous, such as temperature, time, heating/cooling rate, have a significant impact on it, and the stable amorphous can be obtained by optimizing the preparation conditions. For example, the cooling rate will affect the stability of amorphous products when the amorphous products are prepared by melt-quenching method, so it is necessary to select the appropriate cooling rate. The heat generated during the ball milling process will lead to the instability of the amorphous drug. The low temperature ball milling method can be used to reduce the heat generated during the ball milling process, reduce the molecular fluidity and the possibility of crystal transformation. Ball milling time is also a key parameter. Trasi et al. Prepared amorphous griseofulvin, indomethacin, piroxicam, ketoconazole and carbamazepine by low temperature ball milling, and found that with the extension of milling time, the intensity of crystal diffraction peaks of several drugs in the X-ray diffraction pattern gradually decreased until they completely disappeared and formed amorphous forms[91]. However, the amorphous crystal prepared by ball milling may not be completely destroyed, and it is more likely to undergo crystal transformation than the amorphous crystal prepared by other methods. In this case, other methods such as melt-quenching method and solvent evaporation method can be selected.
The addition of some excipients, such as surfactants, antiplasticizers and crystallization inhibitors, can effectively inhibit the amorphous transformation. For example, both magnesium stearate and microcrystalline cellulose inhibit the crystal transformation of amorphous spray-dried lactose[92]. Overhoff et al. Found that sodium dodecyl sulfate stabilized the amorphous supersaturated solution of tacrolimus below the critical micelle concentration and inhibited crystal growth by adsorbing on the surface of the crystal embryo[93]. Many studies have shown that compression can promote amorphous transformation, while the physical barrier caused by excipients can effectively inhibit compression-induced nucleation. For example, amorphous sucrose and etoricoxib can be compression-induced crystallization, but the preparation of solid dispersions with PVP can effectively solve this problem[94,95]. Physical blending of amorphous griseofulvin with HPMCAS, microcrystalline cellulose (MCC), or PEG improves the physical stability of griseofulvin during tableting[96].
When choosing the production process of amorphous preparations, we can predict whether the preparation process-induced crystal transformation will occur according to the results of pre-prescription studies, and design appropriate preparation processes to avoid the occurrence of crystal transformation. For drugs sensitive to moisture or solvent, dry granulation or hot melt granulation can be selected; For drugs that are thermally stable but may undergo crystal transformation during grinding or extrusion, hot melt extrusion granulation can be chosen. For the drugs with moisture and heat instability, the direct powder compression process can be selected; For drugs that cannot be compressed, they can be prepared as capsules rather than tablets.
Polymeric film coatings are also an effective means of increasing the physical stability of the amorphous form, even with very thin coatings (e.g., only a few nanometers thick)[97]. Novakovic et al. Found that with the increase of the thickness of the polymer coating layer, the stability of the amorphous drug also increased significantly[98].
5.5 Other
In recent years, the application of mesoporous carriers has been developed in the pharmaceutical field, which can be used to stabilize amorphous drugs[99]. Porous excipients such as porous silicon or silicate are increasingly used for the delivery of amorphous drugs. Drug molecules are adsorbed in the nanoscale pores of the carrier. Due to the limitation of space, the molecular mobility is reduced and the crystallization kinetics is significantly changed[100]. For example, when fenofibrate is distributed in nanopores with diameters ranging from 18 to 100 nm, the amorphous crystallization rate slows down with the decrease of the pore size[101].
In addition, it has been found that the stability of drugs can be effectively improved by preparing two different cyclodextrins with amorphous drugs to form complexes. Limaprost, α-cyclodextrin and β-cyclodextrin were prepared into complexes by freeze-drying, and the amorphous stability was improved due to the formation of stable ternary inclusion complexes[102,103].
6 Conclusion and prospect
Compared with crystalline drugs, amorphous drugs have the advantages of high apparent solubility and fast dissolution rate due to their high energy state, but they are easy to transform to stable crystalline state during production, storage and dissolution, which ultimately leads to the decrease of oral bioavailability[100]. Studies on that theory of drug amorphous crystallization and the influence factors and inhibit strategies of amorphous crystal transformation, on the one hand, are helpful for pharmaceutical researchers to develop the prediction methods/models of amorphous physical stability and the stabilization method of amorphous,And on that other hand, the invention can provide certain theoretical and technical guidance for the application of the amorphous form of the drug in solid oral preparation.
Stable amorphous preparation products are inseparable from the control of each process of production, quality control and supervision, such as the establishment of polymorphic inspection items in the quality standards of APIs and/or preparations. In July 2007, the FDA issued technical guidelines for generic drug polymorph studies, providing recommendations for monitoring and control of polymorphs in drug substances and/or drug products. The 2015 edition of the Chinese Pharmacopoeia has also added the "Guiding Principles for the Study of Drug Crystal Forms and the Quality Control of Crystal Forms", which provides a reference for the detection of drug crystal forms and the effective control of quality standards, while the 2020 edition of the Chinese Pharmacopoeia has further revised the guiding principles, which shows its importance. It is believed that in the future, with the continuous improvement of the industrial production level of amorphous preparations, the relevant laws and regulations of amorphous drugs will be constantly improved, so as to achieve the purpose of improving the quality of amorphous drugs and ensuring clinical efficacy.
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