Disclosed are compound of formula I whereinR represents hydrogen, lower alkyl, cycloalkyl, bicycloalkyl, adamantyl, aryl, biaryl, or mono- or di-(cycloalkyl, aryl or biaryl)-lower alkyl, di-(lower alkyl or aryl-lower alkyl)amino-lower alkyl, or (piperidino, morpholino, pyrrolidino)-lower alkyl;R.sub.1 represents hydrogen, lower alkyl, cycloalkyl, aryl, biaryl, or (cycloalkyl, aryl or biaryl)-lower alkyl;R.sub.2 represents hydrogen, lower alkyl, lower alkoxy, aryl-lower alkyl, aryl-lower alkoxy, amino, mono- or di-(lower alkyl or aryl-lower alkyl)-amino, acylamino, or (lower alkyl or aryl-lower alkyl)-(thio, sulfinyl or sulfonyl);R.sub.3 represents hydrogen, lower alkyl, cycloalkyl, aryl-lower alkyl, cycloalkyl-lower alkyl, or C.sub.2 -C.sub.7 -alkyl interrupted by S, SO, SO.sub.2, O or N--R.sub.5 ;R.sub.4 represents hydrogen or acyl;R.sub.5 represents hydrogen, lower alkyl, aryl-lower alkyl, acyl, or (lower alkyl, aryl or aryl-lower alkyl)-sulfonyl;A together with the carbon to which it is attached forms a ring and represents a bivalent radical of the formula (CH.sub.2).sub.P which may be interrupted by S, SO, SO.sub.2, O, or N--R.sub.5 ;n represents an integer from zero to four;p represents an integer from 2 to 6;any pharmaceutically acceptable salts thereof;and disulfides corresponding to said compounds of formula I wherein R.sub.4 is hydrogen;methods for preparation thereof;pharmaceutical compositions comprising said compounds; and a method of inhibiting matrix-degrading metalloproteinases in mammals using such compounds.