The hydrophilic poly[N-(2-hydroxypropyl)
methacrylamide] (PHPMA) was used for
RNase A or
BS-RNase modification to prevent their degradation in bloodstream or fast elimination. Two PHPMA chains (classic and star-like) were synthesized and their conjugates with both
enzymes were tested on the CD-1 nude mice bearing various human
tumors. These
RNase conjugates injected intravenously or intraperitoneally into the mice bearing
melanoma,
neuroblastoma or ovarian
tumor caused significant reduction of transplanted
tumors following ten daily doses of 2.5 and/or 1 mg/kg, respectively, while free
RNase A or
BS-RNase injected in doses of 10 mg/kg exerted only negligible antitumor activity. Histological examination confirmed potent cytotoxic effect of
RNase A conjugates in ovarian
tumor. Despite the antitumor activity observed in vivo, the in vitro cytotoxic activity of
RNase A conjugates was not pronounced and did not differ from that caused by the free
RNase A. The in vitro experiments with 125I-labeled preparations demonstrated that
polymer conjugates were internalized by
tumor cells very poorly in contrast to the dose-dependent internalization of the wild
enzyme preparation. Surprisingly, mice injected with EL-4 leukemic cells, which were preincubated for 4 h with
BS-RNase conjugates, exerted significantly prolonged survival compared with the control non-treated mice. It may be supposed that both
BS-RNase and
RNase A conjugates with PHPMA act after administration in vivo by a mechanism different from that or those occurring under in vitro conditions because in vivo they exert an antitumor action, whereas in vitro, they are ineffective. The experiments proved that
RNase A, when conjugated to PHPMA, produced identical aspermatogenic and antitumor effects as
BS-RNase conjugated to this
polymer and that this preparation may be regarded as a potential anticancer
drug.