Solid State Characterization of Dehydroepiandrosterone zyx LUH-CHIAN CHANG~, zyxwvutsrq MINO R. CAIRA~, AND J. KEITH GUILLORY~~ Received April 10, 1995, from the zyxwvutsrq tCollege of Pharmacy, The University of Iowa, Iowa City, IA 52242, andtDepatIment of Chemistry, University of Capetown, Private Bag, Rondebosch 7700, South Africa. Accepted for publication June 27, 1995@. ~ Abstract 0 Three polymorphs (forms 1-Ill), a monohydrate (form S2), and three new solvates [4:1 hydrate (form SI), monohydrate (form S3), and methanol half-solvate(form zyxwvutsr S4)] were isolated and characterized by X-ray powder diffractometry (XRPD), IR spectroscopy, differential scanning calorimetry (DSC), hot stage microscopy, solution calorimetry, and their dissolution rates. A new polymorph, designated as form V, melting at 146.5-148"C, was observed by hot stage microscopy. Our results indicate that only forms I and S4 exhibit reproducible DSC thermograms. Five of the isolated modifications undergo phase transformation on heating, and their DSC thermograms are not reproducible. Interpretation of DSC thermograms was facilitated by use of hot stage microscopy. The identification of each modification is based on XRPD patterns (except forms 53 and 54, for which the XRPD patterns are indistinguishable) and IR spectra. In the IR spectra, a significant difference was observed in the OH stretching region for all seven modifications. In a purity determination study, 5% of a contaminant modification in binary mixtures of several modifications could be detected by use of XRPD. To obtain a better understanding of the thermodynamic properties of these modifications, a series of increasing heating rates and different pan types were used in DSC. According to Burger's rule, forms 1-111 are monotropic polymorphs with decreasing stability in the order form I > form II > form 111. The melting onsets and heats of fusion for forms 1-111 are 149.1 "C, 25.5 kJ/mol; 140.8 "C, 24.6 kJ/mol; and 137.8 "C, 24.0 kJ/mol, respectively. For form 111 the heat of fusion was calculated from heat of solution and DSC data. In the case of form S1 the melting point, 127.2"C, was obtained by DSC using a hermetically sealed pan. The relative stabilities of the six modifications stored under high humidity conditions were predicted to be, on the basis of the heat of solution and thermal analysis data, form S2 > form S3 > form S1 > form I > form II > form 111. However,the results of the dissolution rate determination were inconsistent with the heat of solution data. The stable form I shows a higher initial dissolution rate than the metastable form II and unstable form Ill. All modifications were converted into the stable monohydrate, form S2, during the dissolution study, suggesting that the moisture level in solid formulations should be carefully controlled. Introduction Dehydroepiandrosterone [DHEA, prasterone, or 3p-hy- droxyandrost-5-en-17-one zyxwvutsrqp (l)] was previously ignored as a relatively inert adrenal steroidogenic intermediate. An in- creasing number of studies, in both human subjects and experimental animals, indicate that DHEA has significant physiologic effects; e.g., it may inhibit tumor formation,l provide protection against osteoporosis,2 increase immune system ~ompetence,~ and lower cardiovascular mortality in men.4 Indeed, the clinical importance of DHEA is so great that it has been referred to as the "Mother Steroid" in a recent re vie^.^ However, this compound is practically insoluble in water (-25 ,ug/mL a t 36 zyxwvuts f 1 "C). Therefore, efforts have been exerted to increase its water solubility or dissolution rate and @Abstract published in zyxwvutsrqpo Advance ACS Abstracts, August 1, 1995. 0 HO w 1 to obtain bioavailable formulations. Two approaches that have been employed are the use of the more soluble sodium sulfate derivative and of matrices such as PLA.6 Polymor- phism is another area of investigation for the improvement of pharmaceutical formulations. Since polymorphs have different physical and chemical proper tie^,^ including solubil- ity and dissolution rate, they may also exhibit differences in bioavailability . The Merck Indexs suggests that DHEA is dimorphous, with needles melting at 140-141 "C and leaflets at 152-153 "C. Kuhnert-Brandstatter has reported that four modifications of DHEA can be observed by thermomicr~scopy.~ Cox, et a1.,I0 obtained a monohydrate of DHEA during a steroid synthesis and also determined its crystal structure, but no further details on the polymorphism of DHEA have been reported. In this study, we have isolated three polymorphs and four solvates of the compound and have characterized and identi- fied each form utilizing X-ray powder diffractometry, thermal analysis, infrared spectral analysis, and hot stage microscopy. In addition, the relative stabilities of the various modifications were determined by studying their heats of solution and dissolution rates. Experimental Section Materials-DHEA was purchased from Sigma Chemical Co. (St. Louis, MO). The solvents used were glass-distilled water, acetone, acetonitrile, ethyl acetate, 2-propanol, dioxane, tetrahydrofuran, chloroform, and methanol, all of analytical reagent grade. Preparation of Polymorphs-Crystals of form I were prepared by dissolving excess DHEA in ethyl acetate, acetone, acetonitrile or 2-propanol with the aid of heat. The crystals were collected after the solutions were cooled and agitated in a closed system at room temperature overnight or after allowing the solvents to evaporate very slowly under vacuum conditions. Form I1 crystals were obtained by dissolving DHEA in dioxane, tetrahydrofuran (THF), chloroform, or in the cosolvents of chloroform and THF. The crystals were collected after allowing the solvent to evaporate rapidly at room temperature under vacuum conditions. Form I11 crystals were obtained by desolvation of forms S3 or S4 at room temperature under vacuum conditions. Crystals of form S1, a 4:l hydrate, were obtained from methylene chloride after allowing the solvent to evaporate slowly under condi- tions of 50-60% relative humidty (RH) at room temperature. Crystals of form S2, a monohydrate, were obtained by dissolving excess DHEA in hot 40% ethanol solution and then cooling at room temperature or by suspending powdered DHEA in distilled water at 37 "C for 1 day. Form S2 was also obtained by slow evaporation of the solvent in an ambient atmosphere from dilute DHEA solutions prepared using acetonitrile, acetone, ethyl acetate, or THF. This procedure also results in formation of a monohydrate since the moisture from the atmosphere initiates crystallization of form S2. 0 1995, American Chemical Society and American Pharmaceutical Association 0022-3549/95/3 184- 1 169$09.00/0 Journal of Pharmaceutical Sciences / 1169 Vol. 84, No. 10, October 1995