The present invention provides an assay for determining the biochemical fitness of a biochemical species in a mutant replicating biological entity relative to its predecessor. The present invention further provides a continuous fluorogenic assay for measuring the anti-HIV protease activity of protease inhibitor. The present invention also provides a method of administering a therapeutic compound that reduces the chances of the emergence of drug resistance in therapy. The present invention also provides a compound of formula (I) or a pharmaceutically acceptable salt, a prodrug, a composition, or an ester thereof, wherein A is a group of formulas (A), (B), (C) or (D); R.sup.1, R.sup.2, R.sup.3, R.sup.5 or R.sup.6 is H, or an optionally substituted and/or heteroatom-bearing alkyl, alkenyl, alkynyl, or cyclic group; Y and/or Z are CH.sub.2, O, S, SO, SO.sub.2, amino, amides, carbamates, ureas, or thiocarbonyl derivatives thereof, optionally substituted with an alkyl, alkenyl, or alkynyl group; n is from 1 to 5; X is a bond, an optionally substituted methylene or ethylene, an amino, O or S; Q is C(O), C(S), or SO.sub.2; m is from 0 to 6; R.sup.4 is OH, .dbd.O (keto), NH.sub.2, or alkylamino, including esters, amides, and salts thereof; and W is C(O), C(S), S(O), or SO.sub.2. Optionally, R.sup.5 and R.sup.6, together with the N--W bond of formula (I), comprises a macrocyclic ring.