Research Article Investigating the Physicochemical Stability of Highly Purified Darunavir Ethanolate Extracted from PREZISTA® Tablets Haichen Nie, 1,3 Huaping Mo, 2 and Stephen R. Byrn 1,3 Received 21 March 2018; accepted 27 April 2018 Abstract. Understanding physicochemical stability of darunavir ethanolate is expected to be of critical importance for the development and manufacturing of high-quality darunavir- related pharmaceutical products. However, there are no enabling monographs for darunavir to illustrate its solid-state chemistry, impurity profile, and assay methods. In addition, the US Pharmacopeia reference standard of darunavir is still not commercially available. It has been also challenging to find reliable vendors to obtain highly purified darunavir ethanolate crystals to conduct the physicochemical stability testing. In the present research, we developed a straightforward and cost-effective approach to extract and purify darunavir ethanolate from PREZISTA® tablets using reverse-engineering and crystallization. Using these highly purified crystals, we thoroughly evaluated the potential risks of degradation and form conversions of darunavir ethanolate at stressed conditions to define the manufacturing and packaging specifications for darunavir-related products. Amorphization was observed under thermal storage caused by desolvation of darunavir ethanolate. The ethanolate-to- hydrate conversion of darunavir was observed at high relative humidity conditions. Moreover, acid/base-induced degradations of darunavir have been investigated herein to determine the possible drug-excipient compatibility issues in formulations. Furthermore, it is of particular interests to allow the production of high-quality darunavir-ritonavir fixed dose combinations for marketing in Africa. Thus, a validated HPLC method was developed according to ICH guideline to simultaneously quantify assays of darunavir and ritonavir in a single injection. In summary, the findings of this study provide important information for pharmaceutical scientists to design and develop reliable formulations and processings for darunavir-related products with improved stability. KEY WORDS: darunavir ethanolate; darunavir hydrate; desolvation; form conversion; darunvair ritonavir combination. INTRODUCTION Protease inhibitors (PIs) of human immunodeficiency virus (HIV) are widely acknowledged as one of the most potent antiretroviral medications by targeting at HIV-1 reverse tran- scriptase to inhibit the processing of viral precursor proteins (1,2). However, the limitation of applying many PIs is the rapid development of drug-resistant HIV-1 variants in the viral reservoirs remaining in the infected tissue of patients (3,4). Darunavir (DRV), a second-generation synthetic non-peptidic protease inhibitor, has been applied in treating HIV infections with drug resistance of other PIs especially for patients with a long-term antiretroviral therapy (5–7). Compared to other PIs, DRV not only can inhibit the dimerization of HIV protease enzyme but also can block the cleavage of the natural peptide substrate, which could further abolish the virus replication effectively and safely (8). Therefore, tablet containing 600 mg DRV (PREZISTA®) has been designed and formulated for oral delivery, which has been approved by FDA as a prescribed anti-HIV medication in clinical practice (9–11). Moreover, it has been reported that DRV can be safely administrated with a reduced dose (15 mg/kg) to pediatric population above 3 years (12). DRV ethanolate, a solvate of the drug substance in solid state, was selected as its crystalline form in the PREZISTA® tablets. Apart from the ethanolate crystals, DRV is also reported to exist in its hydrate form and non-solvated/ Electronic supplementary material The online version of this article (https://doi.org/10.1208/s12249-018-1036-x) contains supplementary material, which is available to authorized users. 1 Department of Industrial and Physical Pharmacy, Purdue Univer- sity, 575 Stadium Mall Drive, West Lafayette, Indiana 47907, USA. 2 Department of Medicinal Chemistry and Molecular Pharmacology, Purdue University, 575 Stadium Mall Drive, West Lafayette, Indiana 47907, USA. 3 To whom correspondence should be addressed. (e –mail: nie7@purdue.edu; sbyrn@purdue.edu) AAPS PharmSciTech ( # 2018) DOI: 10.1208/s12249-018-1036-x 1530-9932/18/0000-0001/0 # 2018 American Association of Pharmaceutical Scientists