ANTITUMOR EFFECTS OF AN ANTIBODY-CARBOXYPEPTIDASE G2 CONJUGATE IN COMBINATION WITH PHENOL MUSTARD PRODRUGS

ADEPT is an antibody-based targeting strategy for the treatment of can cer. We have developed two new prodrugs, is(2-chloroethyl)amino]-pheno xycarbonyl-L-glutamic acid (PGP) and ino]-phenoxycarbonyl]amino]-4-(5- tetrazoyl)butyric acid (PTP), which are cleaved by the bacterial enzym e CPG2 to release the 4-[N,N-bis(2-chloroethyl)amino] phenol drug. In vitro, both prodrugs are approximately 100- to 200-fold less potent th an the parent drug (1 h IC50 = 1.4 mu M) in LoVo colorectal tumour cel ls. These prodrugs have been evaluated for utility in ADEPT when used in combination with a conjugate of CPG2 and the F(ab')(2) fragment of the anti-CEA monoclonal antibody, A5B7. The conjugate was shown to loc alise specifically to established LoVo tumour xenografts growing in nu de mice and optimal tumour-normal tissue ratios were achieved after 72 h. Administration of either prodrug, at doses which cause 6-8% body w eight loss, 72 h after administration of the A5B7-CPG2 conjugate to th e LoVo tumour-bearing mice resulted in tumour regressions and growth d elays of 14-28 days. The PTP prodrug in combination with a high dose o f conjugate (10 mg kg(-1)) gave the best anti-tumour activity despite being a 10-fold worse substrate for CPG2 than PGP. Prodrug alone, acti ve drug alone or prodrug in combination with a non-specific conjugate had minimal anti-tumour activity in this tumour model.