Article pubs.acs.org/molecularpharmaceutics
A Win−Win Solution in Oral Delivery of Lipophilic Drugs: Supersaturation via Amorphous Solid Dispersions Increases Apparent Solubility without Sacrifice of Intestinal Membrane Permeability Jonathan M. Miller,† Avital Beig,‡ Robert A. Carr,† Julie K. Spence,† and Arik Dahan*,‡ †
Global Pharmaceutical Research and Development, Abbott Laboratories, Abbott Park, Illinois 60064, United States Department of Clinical Pharmacology, School of Pharmacy, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
‡
ABSTRACT: Recently, we have revealed a trade-off between solubility increase and permeability decrease when solubilityenabling oral formulations are employed. We have shown this trade-off phenomenon to be ubiquitous, and to exist whenever the aqueous solubility is increased via solubilizing excipients, regardless if the mechanism involves decreased free fraction (cyclodextrins complexation, surfactant micellization) or simple cosolvent solubilization. Discovering a way to increase drug solubility without concomitant decreased permeability represents a major advancement in oral delivery of lipophilic drugs and is the goal of this work. For this purpose, we sought to elucidate the solubility−permeability interplay when increased apparent solubility is obtained via supersaturation from an amorphous solid dispersion (ASD) formulation. A spray-dried ASD of the lipophilic drug progesterone was prepared in the hydrophilic polymer hydroxypropyl methylcellulose acetate succinate (HPMCAS), which enabled supersaturation up to 4× the crystalline drug's aqueous solubility (8 μg/mL). The apparent permeability of progesterone from the ASD in HPMC-AS was then measured as a function of increasing apparent solubility (supersaturation) in the PAMPA and rat intestinal perfusion models. In contrast to previous cases in which apparent solubility increases via cyclodextrins, surfactants, and cosolvents resulted in decreased apparent permeability, supersaturation via ASD resulted in no decrease in apparent permeability with increasing apparent solubility. As a result, overall flux increased markedly with increasing apparent solubility via ASD as compared to the other formulation approaches. This work demonstrates that supersaturation via ASDs has a subtle yet powerful advantage over other solubility-enabling formulation approaches. That is, increased apparent solubility may be achieved without the expense of apparent intestinal membrane permeability. Thus, supersaturation via ASDs presents a markedly increased opportunity to maximize overall oral drug absorption. KEYWORDS: amorphous solid dispersions, low-solubility drugs, solubility−permeability interplay, drug transport analysis, intestinal permeability, oral drug absorption
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INTRODUCTION The rate and extent of drug absorption from the gastrointestinal (GI) tract are very complex and affected by many factors, including physicochemical, physiological, and dosage form related factors.1−3 Despite this complexity, the work of Amidon et al.4 revealed that the fundamental parameters controlling oral drug absorption are the permeability of the drug through the GI membrane and the solubility/dissolution of the drug dose in the GI milieu. These key parameters are characterized in the Biopharmaceutics Classification System © 2012 American Chemical Society
(BCS), one of the most significant available modern tools to facilitate oral drug product development.5−8 As a result of modern drug discovery techniques, the number of low-solubility drug candidates is continuously increasing, and by some estimates, more than 50% of new drug candidates are lipophilic and exhibit poor aqueous solubility.9−12 Dissolution Received: Revised: Accepted: Published: 2009
February 21, 2012 April 17, 2012 May 27, 2012 May 27, 2012 dx.doi.org/10.1021/mp300104s | Mol. Pharmaceutics 2012, 9, 2009−2016
Molecular Pharmaceutics
Article
GI milieu partitioning may not lead to the undesired trade-off when using amorphous solid dispersions. The purpose of this research was to elucidate the solubility− permeability interplay when increased apparent solubility is obtained via supersaturation from an amorphous solid dispersion (ASD) formulation. A spray-dried ASD of the highly lipophilic, BCS class II drug progesterone was prepared in the hydrophilic polymer hydroxypropyl methylcellulose acetate succinate (HPMC-AS), the degree of supersaturation vs time was evaluated, and the apparent permeability of the drug from the ASD formulation was measured as a function of increasing apparent solubility (supersaturation) in the PAMPA and rat intestinal perfusion models. Moreover, a comparison between the ASD and the previously studied formulations (cyclodextrins, surfactants, cosolvents) was performed. Overall, this research reveals a novel perspective and provides significant insights in the field of oral delivery of low-solubility drugs.
of the drug substance in the aqueous GI milieu is almost always a prerequisite for absorption following oral administration, and therefore, inadequate aqueous solubility often results in limited oral bioavailability. In today’s world, thus, low aqueous solubility is a common problem plaguing the drug candidates in the pipeline of most major pharmaceutical companies. A wide variety of solubility-enabling formulation approaches have been developed and are routinely used to tackle the problem of inadequate aqueous solubility, e.g., the use of surface active agents, lipid-based formulations, self-emulsifying drug delivery systems, cyclodextrins, cosolvents, amorphous solid dispersions, and other techniques. While a significant increase in the drug’s apparent solubility may certainly be achieved by these techniques, their effects on the drug’s intestinal permeability, the key parameter (alongside solubility) that governs oral absorption, is usually poorly understood and often overlooked. Intestinal permeability refers to the flow of a substance across the organ: how deep can a substance penetrate into the intestinal wall per time unit? Mathematically, permeability is equal to the diffusion coefficient of the drug through the membrane times the membrane/aqueous partition coefficient of the drug divided by the membrane thickness. The direct correlation between the intestinal permeability and the drug’s GI membrane/GI milieu partitioning, which in turn is dependent on the drug’s apparent solubility in the GI milieu, suggests that the two key parameters dictating oral drug absorption, the solubility and the permeability, may be closely associated, and may exhibit an interplay between them. Indeed, we have previously shown that a trade-off exists between apparent solubility increase and permeability decrease when using solubility enabling formulations.13,14 We have demonstrated this phenomenon for systems involving complexation,13,15 micellar solubilization,14 and systems involving no decreased free fraction of the drug, i.e., cosolvents,16,17 showing the direct interplay and trade-off between these two key parameters. Altogether, these findings indicated that a price is paid when using solubility-enabling formulations; an inevitable decreased apparent permeability accompanied the solubility increase, which may jeopardize the overall drug exposure.18 Discovering a way to increase drug solubility without concomitant decreased permeability represents a major advancement in the field of oral drug delivery, research, and development and is the goal of this paper. From the large variety of solubility-enabling techniques, we have pointed out amorphous solid dispersions (ASD) as a potential win−win method, i.e., a method that will enable increased apparent solubility without hampering the permeability. The amorphous state has long been recognized as a way to increase the free energy and the apparent aqueous solubility of poorly soluble pharmaceuticals.19−21 Over the past few decades, amorphous solid dispersion technologies have emerged to enable stabilization of the amorphous state both in the dosage form and during supersaturation in the GI milieu.22−26 In fact, delivery of some of the most important drugs of the twenty-first century has been made possible through amorphous solid dispersion technologies, e.g., Kaletra (Abbott), Sporanox (Janssen), Norvir (Abbott), and others. As opposed to other solubilization methods, the use of amorphous solid dispersions does not alter the equilibrium solubility of the drug; rather, it enables an unstable supersaturated solution to be attained. Thus, we hypothesized that the solubility−permeability linkage through the GI membrane/
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THEORY The quasi-equilibrium analyses of the effect of increased apparent solubility by cyclodextrins, surfactants, and cosolvents on apparent membrane permeability has been described in detail previously.13,14,16,17 The fundamental equations are briefly summarized here. The apparent membrane permeability (Pm) dependence on drug apparent aqueous solubility (Saq) can be written as Pm =
Pm(o)Saq(o) Saq
(1)
where Pm(o) is the intrinsic membrane permeability of the drug and Saq(o) is the intrinsic aqueous solubility of the drug in the absence of solubilizer. It is important to note that Saq represents the apparent equilibrium solubility in the presence of solubilizer and not the apparent kinetic (i.e., supersaturation) solubility. Likewise, the apparent unstirred aqueous boundary layer permeability (Paq) dependence on Saq can be written as
Paq =
Paq(o)Daq Daq(o)F
(2)
where F = Saq(o)/Saq. The overall effective permeability (Peff) of the drug can be written as 1 Peff = 1 1 + P P aq
m
(3)
Thus, the overall Peff dependence on drug apparent solubility and solubilizer concentration may be predicted via eq 3 wherein the Pm and Paq dependence on Saq are predicted using eqs 1 and 2 with knowledge of Saq(o), Saq, Pm(o), Paq(o), Daq(o), and Daq.
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MATERIALS AND METHODS Materials. Progesterone, MES buffer, and trifluoroacetic acid (TFA) were purchased from Sigma Chemical Co. (St. Louis, MO). Hydroxypropyl methylcellulose acetate succinate (HPMC-AS) LF grade was obtained from Shin-Etsu Chemical Co. (Tokyo, Japan). KCl and NaCl were obtained from Fisher Scientific Inc. (Pittsburgh, PA). Acetonitrile and water (Merck KGaA, Darmstadt, Germany) were UPLC grade. All other chemicals were of analytical reagent grade. Preparation of Amorphous Solid Dispersion (ASD) by Spray Drying. The amorphous solid dispersion powder of 5%
2010
dx.doi.org/10.1021/mp300104s | Mol. Pharmaceutics 2012, 9, 2009−2016
Molecular Pharmaceutics
Article
MA) 96-well MultiScreen-Permeability filter plates with 0.3 cm2 polycarbonate filter support (0.45 mm). The filter supports in each well were first impregnated with 15 μL of a 5% (v/v) hexadecane in hexane solution.31 The wells were then allowed to dry for 1 h to ensure complete evaporation of the hexane. The donor wells were then loaded with 0.2 mL of the supersaturated progesterone solutions, and each receiver well was loaded with 0.3 mL of blank MES buffer. Four wells were loaded at each PEG-400 level to enable collection at different time points. Each experiment was repeated six times (n = 6). The donor plate was then placed upon the 96-well receiver plate, and the resulting PAMPA sandwich was incubated at room temperature (25 °C). Receiver plate wells were then collected every 30 min over two hours, and the progesterone concentration in each well was determined by UPLC. Permeability coefficient (Papp) was calculated from the linear plot of drug accumulated in the receiver side versus time, according to the equation
progesterone in HPMC-AS27 was prepared by spray drying. Drug and polymer were dissolved in acetone at a total solids load of 5% (w/v). This solution was then spray-dried using a Buchi (Flawil, Switzerland) mini-spray dryer B-290 at an inlet temperature of 80 °C, outlet temperature of approximately 45 °C, and feed solution infusion rate of approximately 5 mL per minute. The size range for the resulting spray-dried particles was approximately 10−20 μm. Powder X-ray Diffraction (PXRD). Powder X-ray diffraction (PXRD) was carried out on a theta/theta diffractometer (model Ultima II D/Max-2000-PC with model SA-HF3 3 kW X-ray generator, controlled by basic DMax2200PC series software, Rigaku Corp., Tokyo, Japan). X-rays were generated using a Cu anode with a generator power of 50 kV and 40 mA. Approximately 20 mg of each sample was pressed flat onto custom zero background silicon disks (Gem Dugout, State College, PA). Samples were scanned from 5 to 40° 2θ at 2° 2θ/. Data were processed using Jade software (version 6.5 or 7.0, Materials Data, Inc., Livermore, CA). Modulated Differential Scanning Calorimetry (mDSC). Modulated DSC was carried out on a TA Instruments (New Castle, DE) Q100 DSC. Samples were prepared by weighing about 7 mg into an aluminum pan, which was then covered with a pierced aluminum lid. The samples were then heated from 25 to 150 °C at 3 °C/min under N2 purge at 50 mL/min. The temperature was modulated with amplitude of ±1 °C and period of 60 s. Determination of Supersaturation vs Time for Progesterone ASD. The solution stability of supersaturated solutions prepared from the progesterone ASD was determined according to previously reported methods28,29 with minor adaptations. All experiments were carried out at 37 °C in triplicate in 20 mL of aqueous solution. Briefly, supersaturated solutions of progesterone were obtained by dissolving an appropriate amount of the 5% (w/w) progesterone ASD powder into MES buffer pH 6.5 to achieve supersaturated solution of 2× (16 μg/mL), 4× (32 μg/mL), 6× (48 μg/mL), and 8× (64 μg/mL) the equilibrium solubility of crystalline progesterone (8 μg/mL). The resulting supersaturated solutions were then allowed to stand at 37 °C with no agitation. The supersaturated solutions were then periodically sampled, and progesterone concentration was determined by UPLC. Periodic sampling was carried out by withdrawing approximately 200 μL and separating any solids by centrifugation prior to assay by UPLC. The supersaturated solutions were assayed for progesterone concentration for at least 10 h, which would provide sufficient time to carry out permeability studies in the PAMPA and rat intestinal perfusion models. As a control study to demonstrate true supersaturation in these experiments, the equilibrium solubility of crystalline progesterone was measured as a function of HPMC-AS concentration in the range of weight % polymer used in the supersaturation studies (0.015−0.15%) and found to be unchanged by the presence of the polymer. Parallel Artificial Membrane Permeability Assay (PAMPA). Studies of the permeability through artificial membrane were carried out using the hexadecane-based PAMPA assay, as described previously.13,30 Briefly, various degrees of supersaturated solutions (0.5−4 times equilibrium solubility) were prepared from an amorphous solid dispersion formulation of progesterone with 10 mM MES buffer pH 6.5. PAMPA experiments were carried out in Millipore (Danvers,
Papp =
dQ /dt A ·C0
where dQ/dt is the steady-state appearance rate of progesterone on the receiver side, C0 the initial concentration of the drug in the donor side, and A the membrane surface area (0.048 cm2). Linear regression was carried out to obtain the steadystate appearance rate of progesterone on the receiver side. Single-Pass Intestinal Perfusion Studies in Rats. All animal experiments were conducted using protocols approved by the Ben-Gurion University of the Negev Animal Use and Care Committee (Protocol IL-60-11-2010). Animals were housed and handled according to the Ben-Gurion University of the Negev Unit for Laboratory Animal Medicine Guidelines. Male Wistar rats (Harlan, Israel) weighing 250−280 g were used for all studies. Prior to each experiment, the rats were fasted overnight (12 h) with free access to water. Animals were randomly assigned to the different experimental groups. The procedure for the single-pass in situ jejunal perfusions followed previous reports.7,32,33 Briefly, rats were anesthetized (1 mL/kg ketamine−xylazine 9%:1%) and placed on a 37 °C surface (Harvard Apparatus Inc., Holliston, MA). A proximal jejunal segment of approximately 10 cm was carefully exposed and cannulated on two ends with silicone tubing (WatsonMarlow ltd, Wilmington, MA). Care was taken to avoid disturbance of the circulatory system, and the exposed segment was kept moist with 37 °C normal saline solution. Various degrees of supersaturated solutions (0.5−4 times equilibrium solubility) were prepared from an amorphous solid dispersion formulation of progesterone, in 10 mM MES buffer, pH 6.5, 135 mM NaCl, 5 mM KCl. All perfusate solutions were incubated in a 37 °C water bath and were pumped through the intestinal segment (Watson-Marlow 205S, Wilmington, MA). The isolated segment was first rinsed with blank perfusion buffer at a flow rate of 0.5 mL/min in order to clean out any residual debris. The test solutions were then perfused through the intestinal segment at a flow rate of 0.2 mL/min. The perfusion buffer was first perfused for 1 h, in order to ensure steady state conditions. After reaching steady state, samples were taken at 10 min intervals for an additional one hour. All samples were immediately assayed for drug content by UPLC. Following the termination of the experiment, the length of each perfused jejunal segment was accurately measured. 2011
dx.doi.org/10.1021/mp300104s | Mol. Pharmaceutics 2012, 9, 2009−2016
Molecular Pharmaceutics
Article
Figure 1. PXRD pattern of progesterone crystalline powder (A; left panel) and amorphous solid dispersion in HPMC-AS (B; right panel).
Figure 2. DSC thermogram of progesterone crystalline powder (A; left panel) and amorphous solid dispersion in HPMC-AS (B; right panel).
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RESULTS PXRD and DSC of Progesterone ASD in HPMC-AS. The crystalline API and ASD powders of progesterone were characterized by PXRD and DSC (Figures 1 and 2). The PXRD pattern of the ASD was completely devoid of any diffraction peaks as compared to the crystalline pattern, indicating successful formation of the amorphous solid dispersion in HPMC-AS (Figure 1). As further evidence of amorphicity, the mDSC thermogram of the ASD powder showed a single glass transition temperature of approximately 108 °C, whereas the melting endotherm of crystalline progesterone at approximately 130 °C was absent (Figure 2). Supersaturation Behavior of Progesterone HPMC-AS ASD. Figure 3 shows the stability of supersaturated solutions of progesterone obtained by dissolving the ASD in HPMC-AS powder at concentrations of 2× (16 μg/mL), 4× (32 μg/mL), 6× (48 μg/mL), and 8× (64 μg/mL) the equilibrium solubility of crystalline progesterone (8 μg/mL). Solutions up to 32 μg/ mL maintained supersaturation for at least 10 h, providing sufficient time to carry out permeability studies in the PAMPA and rat intestinal perfusion models. Supersaturated solutions at 48 μg/mL and 64 μg/mL) were only transiently stable for
