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Journal of Virology, May 2007, p. 5144-5154, Vol. 81, No. 10
0022-538X/07/$08.00+0     doi:10.1128/JVI.02706-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Unique Thermodynamic Response of Tipranavir to Human Immunodeficiency Virus Type 1 Protease Drug Resistance Mutations

S. Muzammil,¹ A. A. Armstrong,² L. W. Kang,^2, A. Jakalian,³ P. R. Bonneau,³ V. Schmelmer,⁴ L. M. Amzel,² and E. Freire^1,2*

Department of Biology, The Johns Hopkins University, Baltimore, Maryland 21218,¹ Department of Biophysics and Biophysical Chemistry, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205,² Boehringer Ingelheim (Canada), Ltd., Research & Development, Laval, Québec, H7S 2G5 Canada,³ Nippon Boehringer Ingelheim Co., Ltd., Kawanishi, Japan⁴

Received 7 December 2006/ Accepted 28 February 2007

Drug resistance is a major problem affecting the clinical efficacy of antiretroviral agents, including protease inhibitors, in the treatment of infection with human immunodeficiency virus type 1 (HIV-1)/AIDS. Consequently, the elucidation of the mechanisms by which HIV-1 protease inhibitors maintain antiviral activity in the presence of mutations is critical to the development of superior inhibitors. Tipranavir, a nonpeptidic HIV-1 protease inhibitor, has been recently approved for the treatment of HIV infection. Tipranavir inhibits wild-type protease with high potency (K_i = 19 pM) and demonstrates durable efficacy in the treatment of patients infected with HIV-1 strains containing multiple common mutations associated with resistance. The high potency of tipranavir results from a very large favorable entropy change (–TS = –14.6 kcal/mol) combined with a favorable, albeit small, enthalpy change (H = –0.7 kcal/mol, 25°C). Characterization of tipranavir binding to wild-type protease, active site mutants I50V and V82F/I84V, the multidrug-resistant mutant L10I/L33I/M46I/I54V/L63I/V82A/I84V/L90M, and the tipranavir in vitro-selected mutant I13V/V32L/L33F/K45I/V82L/I84V was performed by isothermal titration calorimetry and crystallography. Thermodynamically, the good response of tipranavir arises from a unique behavior: it compensates for entropic losses by actual enthalpic gains or by sustaining minimal enthalpic losses when facing the mutants. The net result is a small loss in binding affinity. Structurally, tipranavir establishes a very strong hydrogen bond network with invariant regions of the protease, which is maintained with the mutants, including catalytic Asp25 and the backbone of Asp29, Asp30, Gly48 and Ile50. Moreover, tipranavir forms hydrogen bonds directly to Ile50, while all other inhibitors do so by being mediated by a water molecule.

Published ahead of print on 14 March 2007.

Present address: Konkuk University, Department of Advanced Technology Fusion, Hwayang-dong 1 bunji Kwangjin-gu, Seoul, South Korea 143-701.

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