Polymer conjugated bovine seminal ribonuclease inhibits growth of solid tumors and development of metastases in mice

Bovine seminal ribonuclease (BS-RNase) exerts a potent cytotoxic activity w hen administered intratumorally (i.t.) to the nude mice bearing human tumor s. The ineffective treatment with intravenous (i.v.) or intraperitoneal (i. p.) administration led us to the synthesis of polymeric conjugates with BS- RNase to prevent it from degradation in the blood vessel. Hydrophilic poly[ N-(2-hydroxypropyl)methacrylamide] (PHPMA) was us ed for B S-RNase modifica tion and a PHPMA-BS-RNase conjugates were prepared. Classic conjugate (P-BS ) with BS-RNase bound to the polymer by its oligopeptide site chains was pr epared by aminolytic reaction of the polymer precursor bearing reactive est er groups situated in the side chains of polymer, while star-like conjugate (S-BS) was synthesized by the reaction of PHPMA containing end-chain react ive group with BS-RNase in aqueous buffer solution at pH 8. In contrast to the total ineffectiveness of free BS-RNase administered i.v. at a daily dos e 10 mg/kg, application of P-BS and S-BS conjugates at doses 2 mg/kg and 0. 5 mg/kg caused significant inhibition of the growth of human melanoma in nu de mice. On the base of these results the effect of i.v. administered S-BS on the metastatic process and the survival of C57B1/6 inbred mice inoculate d with B16 melanoma cells was investigated. Sixty per cent of mice treated with S-BS (0.5 mg/kg/day) survived 100 days without metastatic foci when th e experiment terminated. The average survival time of the treated groups wa s 75.5 days compared to 32.7 days in the control group. BS-RNase conjugated to water soluble polymers appears to be the first BS RNase preparation whi ch exerts anticancer and antimetastatic activity following its intravenous administration.