Based on the substrate transition state and our strategy to tackle the problem of drug resistance, a series of HIV/FIV protease (HIV /FIV PR) monocyclic inhibitors incorporating a 15- or 17-membered macrocycle with an equivalent P3 or P3' group and a unique unnatural amino acid, (2R, 3S)-3-amino-2-hydroxy-4-phenylbutyric acid, have been designed and synthesized. In addition, based on the structure of TL3 with small P3/P3' group, we have synthesized two conformationally restricted bicyclic inhibitors containing the macrocycle, which mimic the P1/P1'-P3/P3' tripeptide [Phe-Val-Ala] of TL3. We have found that the contribution of the macrocycle in our monocyclic inhibitors is important to the overall activity, but the ring size does not affect the activity to a significant extent. Several inhibitors that were developed in this work, exhibit low nanomolar inhibitory activity against the wild-type HIV/FIV PR and found to be highly effective against some drug-resistant as well as TL3-resistant mutants of HIV PRs. Compound 15, in particular, is the most effective cyclic inhibitor in hand to inhibit FIV replication in tissue culture at a concentration of 1.0 micro g/mL (1.2 microM).