Inhibition of tumor growth by poly(ethylene glycol) derivatives of anti-ErbB2 antibodies

Poly(ethylene glycol) (PEG) modification of substances with antitumor activ ity was shown to enhance penetration into growing solid tumors and extend a ntitumor effects. Accordingly, PEG was introduced as a modifier to two type s of monoclonal antibodies (N12 and L26) specific to the ErbB2 (HER2) oncop rotein. These antibodies suppress the growth of tumors overexpressing ErbB2 (e.g. N87 human tumor) and the effect of PEG on their antitumor activity w as evaluated. Methoxy-PEG-maleimide conjugated to sulfhydryl groups at the hinge region of the antibodies impaired their antibody binding to N87 tumor cells and did not enhance the antitumor inhibitory activity in tumor-beari ng mice. A branched N-hydroxysuccinimide-activated PEG (PEG2), conjugated t hrough amino groups of the protein, was used for binding to the whole antib ody (Ab) or to its monomeric Fab' fragment. When tested against N87 cells i n vitro, the binding activity and antitumor cytotoxic effects of Ab-PEG2 we re mostly preserved. PEG2 modification did not seem to alter the tumor-inhi bitory activity of the antibodies in vivo and the same pattern of tumor dev elopment was observed during the first few weeks following administration. However, the stimulating effects of PEG were observed at later stages of tu mor growth since tumor development was either slowed down or completely arr ested. Furthermore, a second tumor implanted into the same mice during this later stage was significantly or completely inhibited, as compared to resu lts in mice injected with the unmodified antibody. The Fab'-PEG2 monomeric derivative was also shown to be effective in inhibiting the growth of a sec ond tumor. The extended and prolonged enhancing effect of PEG on the antitu mor activity of antibodies or Fab' fragments directed against ErbB2 may be of importance in the treatment of ErbB2-overexpressing neoplasms.