A broad screening program previously identified phenprocoumon (1) as a small molecule template for inhibition of HIV protease. Subsequent modification of this lead through iterative cycles of structure-based design led to the activity enhancements of pyrone and dihydropyrone ring systems (II and V) and amide-based substitution (III). Incorporation of sulfonamide substitution within the dihydropyrone template provided a series of highly potent HIV protease inhibitors, with structure-activity relationships described in this paper. Crystallographic studies provided further information on important binding interactions responsible for high enzymatic binding. These studies culminated in compound VI, which inhibits HIV protease with a Ki value of 8 pM and shows an IC90 value of 100 nM in antiviral cell culture. Clinical trials of this compound (PNU-140690, Tipranavir) for treatment of HIV infection are currently underway.

Peptides that contain difluorostatine and difluorostatone residues have been shown to be potent inhibitors of the aspartyl protease renin. The readily hydrated fluoro ketone is proposed to mimic the tetrahedral intermediate that forms during the enzyme-catalyzed hydrolysis of a peptidic bond. It is suggested that the sp3-hybridized ketal acts as a transition-state analogue renin inhibitor. The fluoro ketone is shown to be a much more effective inhibitor than the corresponding nonfluorinated ketone, which acts as a pseudosubstrate. More lipophilic side chains at the P1 site can enhance the inhibitory potency of the difluorostatine analogue, but this cannot be demonstrated in the difluorostatone series. Additionally, high renin specificity has been shown for a difluorostatone-containing peptide.

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The low oral bioavailability and rapid biliary excretion of peptide-derived HIV protease inhibitors have limited their utility as potential therapeutic agents. Our broad screening program to discover nonpeptidic HIV protease inhibitors had previously identified compound II (phenprocoumon, K(i) = 1 muM) as a lead template. Crystal structures of HIV protease complexes containing the peptide-derived inhibitor I (1-(naphthoxyacetyl)-L-histidyl-5(S)-amino-6-cyclohexyl-3 (R),4(R)-dihydroxy-2(R)-isopropylhexanoyl-L-isoleucine N-(2-pyridylmethyl)amide) and nonpeptidic inhibitors, such as phenprocoumon (compound II), provided a rational basis for the structure-based design of more active analogues. This investigation reports on the important finding of a carboxamide functionally appropriately added to the 4-hydroxycoumarin and the 4-hydroxy-2-pyrone templates which resulted in a new promising series of nonpeptidic HIV protease inhibitors with improved enzyme-binding affinity. The most active diastereomer of the carboxamide-containing compound XXIV inhibited HIV-1 protease with a K(i) value of 0.0014 muM. This research provides a new design direction for the discovery of more potent HIV protease inhibitors as potential therapeutic agents for the treatment of HIV infection.

A model of the conformation of the enzyme-bound inhibitor of human renin suggested the possibility of a gamma-lactam conformational restriction at the P2-P3 site. Synthetic routes to these gamma-lactam dipeptide isosteres and their incorporation into potential renin inhibitors are described. Peptide VIa,b with a gamma-lactam conformational constraint and a hydroxyethylene isostere at the cleavage site inhibited human plasma renin with an IC50 value of 6.5 nM. The flexibility of these syntheses should make available a number of potential enzyme inhibitors with this structural feature for the study of enzyme-bound conformers.

The preparations of sodium 4(S)-[(tert-butyloxycarbonyl)amino]-2,2-difluoro-3(S)- and -3(R)-[(4-methoxyphenyl)amino]-6-methylheptanoates (7a and 7b) from sodium 4(S)-[(tert-butyloxycarbonyl)amino]-2,2-difluoro-3(R)- and -3(S)-hydroxy-6-methylheptanoates (1a and 1b) are described. The key step involves the stereospecific intramolecular displacement via a Mitsunobu reaction for the conversion of a beta-hydroxy hydroxamate to a beta-lactam ring. Compounds 7a and 7b are useful as synthetic intermediates for the preparation of enzyme inhibitors that contain 3(S),4(S)- and 3(R),4(S)-diamino-2,2-difluoro-6-methylheptanoic acid inserts. Angiotensinogen analogues VII and VIII that contain these novel amino analogues of difluorostatine were shown to be inhibitors of the enzyme renin. The alpha,alpha-difluoro-beta-aminodeoxystatine-containing compounds were shown to be weaker inhibitors than the corresponding difluorostatine-containing congeners.

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A structure-activity analysis of peptides containing backbone C alpha-methyl and N alpha-methyl modifications led to the discovery of potent renin inhibitors with high metabolic stability. In vitro, Boc-Pro-Phe-N alpha-MeHis-Leu psi-[CHOHCH2]Val-Ile-Amp (XII) is a potent inhibitor of human plasma renin with IC50 of 0.26 nM. It is a much weaker inhibitor of other aspartic proteases such as porcine pepsin or bovine cathepsin D (IC50 = 6 microM). It was shown not to be degraded by a rat liver homogenate preparation. In vivo, it inhibited plasma renin activity and lowered blood pressure of furosemide-treated cynomolgus monkeys. At a dose of 5 mg/kg iv, the pronounced hypotensive response persisted for greater than 3 h postinfusion.