Antitumor resistance induced by zinostatin stimalamer (ZSS), a polymer-conjugated neocarzinostatin (NCS) derivative. I. Meth A tumor eradication and tumor-neutralizing activity in mice pretreated with ZSS or NCS.

Zinostatin stimalamer (ZSS) is a new anticancer agent derived from neocarzinostatin (NCS), which is synthesized by conjugation of one molecule of NCS and two molecules of poly(styrene-co-maleic acid). ZSS exhibited potent in vitro and in vivo antitumor activity in preclinical experiments, and a clinical trial of the intra-arterial administration of ZSS with iodized oil on hepatocellular carcinoma showed potent antitumor activity. We investigated the effect of ZSS and NCS on antitumor resistance and found that pretreatment with either drug suppressed the growth of MethA tumors in Balb/c mice and induced tumor eradication when given separately by single administration at therapeutic doses between 1 day and 4 weeks before tumor transplantation. The findings that the cytocidal activity of these drugs was not detected in vivo at the time of tumor transplantation and that tumor regression was preceded by a period of transient growth suggested that tumor regression was due to host-mediated antitumor activity induced by these drugs. Pretreatment with ZSS or NCS also suppressed the growth of Colon 26 carcinoma and Sarcoma 180. The finding that NCS showed the same effect as ZSS suggests that poly(styrene-comaleic acid) is not essential for the induction of host-mediated antitumor activity. Furthermore, apo-ZSS, which lacks cytocidal activity, did not induce antitumor activity. From this, it is suggested that the cytocidal effect of ZSS involves the induction of host-mediated antitumor resistance. In athymic Balb/c nu/nu mice, pretreatment with ZSS or NCS did not induce tumor eradication, suggesting that mature T lymphocytes play an important role in tumor eradication. Challenging MethA was rejected without transient growth in mice that had been cured of MethA, but challenging Colon 26 was not, showing that anti-MethA resistance was augmented selectively in the MethA-eradicated mice. Splenocytes from MethA-bearing mice pretreated with the drug showed tumor-neutralizing activity beginning 14 days after tumor transplantation. Tumor-neutralizing activity was only induced after MethA transplantation. The effector cells of this tumor-neutralizing activity were Thy1.2+ T lymphocytes that had been passed through a nylon-wool column, but no significant augmentation of cell-mediated cytotoxic activity of splenocytes from MethA-eradicated mice was observed in vitro.