Enhancement of antibody-directed enzyme prodrug therapy in colorectal xenografts by an antivascular agent

The irregular nature of solid tumor vasculature produces a heterogeneous di stribution of antibody-targeted therapies within the tumor mass, which freq uently results in reduced therapeutic efficacy. We have, therefore, combine d two complementary therapies: Antibody-directed Enzyme Prodrug Therapy (AD EPT), which targets tumor cells, and an agent that selectively destroys tam er vasculature. A single i.p. dose (27.5 mg/kg) of the drug 5,6-dimethylxan thenone-4-acetic acid (DMXAA), given to nude mice bearing the LS174T colore ctal xenograft, destroyed all but a peripheral rim of tumor cells, without enhancing survival, The ADEPT system, in which a pretargeted enzyme activat es a prodrug, consisted of the F(ab')(2) fragment of anti-carcinoembryonic antigen antibody A5B7 conjugated to the bacterial enzyme carboxypeptidase G 2 and the prodrug 4 [(2-chroroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutam ic acid, which was given i.p. in three doses of 500 mg/kg at 72, 84, and 96 h postconjugate administration (25 units of carboxypeptidase G2), The anti body-enzyme conjugate could be selectively retained at approximately twice the control levels by administration of the antivascular agent at the time of optimal conjugate localization within the tumor (20 h post-conjugate adm inistration), as demonstrated by gamma counting, phosphor plate image analy sis, and active enzyme measurement. This resulted in significantly enhanced tumor growth inhibition in groups of six mice, compared to conventional AD EPT therapy, with no concomitant increase in systemic toxicity. In a separa te experiment, aimed at trapping the prodrug within the tumor, a 16-fold in crease over control values was produced (means, 44.8 versus 2.8 mu g/g tumo r) when DMXAA was given 4 h prior to 4-[(2-chloroethyl)(2-mesyloxyethyl)ami no]benzoyl-L-glutamic acid. The therapeutic window was small, with no signi ficant enhancement of prodrug retention when DMXAA was given at either earl ier or later time points. This correlated with the time of vascular shut-do wn induced by the antivascular agent. We are currently investigating whethe r it is more advantageous to trap increased levels of conjugate or prodrug within the tumor for maximal enhancement of conventional ADEPT, These studi es demonstrate that combined use of antibody-directed and antivascular ther apies can significantly benefit the therapeutic outcome of either strategy alone.