Wortmannin, a phosphoinositide 3-kinase inhibitor, selectively enhances cytotoxicity of receptor-directed-toxin chimeras in vitro and in vivo

Background. Generalized resistance of some neoplastic cell lines to treatme nt with ligand-toxin chimeras has been attributed to an increased rate of l ysosomal uptake and degradation following endocytosis of the chimera-recept or complex. Because phosphoinositide S-kinase (PI 3-kinase) activity is kno wn to play a role in intracellular trafficking, particularly from endosomes to lysosomes, we hypothesized that co-exposing cells to the PI 3-kinase in hibitor, wortmannin, might enhance cytotoxicity of ligand-toxin chimeras. M ethods. In vitro, cytotoxicity of five receptor directed-toxin chimeras (bF GF-SAP, bFGF-PE, aFGF-PE, HBEGF-SAP, bFGF-gelonin) and an immunotoxin (11A8 -SAP) was examined in the presence or absence of this PI 3-kinase inhibitor against a panel of human neoplastic cell lines: SK-MEL-5 (melanoma), PA-I (ovarian teratocarcinoma), DU145 (prostatic carcinoma) and MCF-7 (breast ca rcinoma). In vivo antitumor activity of a treatment regimen combining wortm annin (1 or 2 mg/kg i.p.) and bFGF-SAP (10 mu g/kg i.v.) once a week for 4 weeks was evaluated compared to administration of each agent alone in C3H/H eN mice implanted with the FSallC murine fibrosarcoma. Results. At concentr ations greater than the reported K-i for PI S-kinase inhibition (1-10 mu M) , wortmannin enhanced cytotoxicity when combined with saporin or gelonin ch imeras, but produced subadditive cytotoxicity when combined with Pseudomona s exotoxin chimeras. When low nanomolar concentrations selective for PI 3-k inase inhibition (5-100 nM) were examined for effects on one receptor direc ted-toxin chimera, wortmannin dramatically enhanced bFGF-SAP cytotoxicity i n three of the four cell lines. A different PI S-kinase inhibitor, LY294002 (K-i similar to 1 mu M), however, failed to potentiate bFGF-SAP. When admi nistered to mice, wortmannin combined with bFGF-SAP resulted in a significa nt decrease in tumor volumes compared to vehicle-treated controls that was not observed in mice treated with either agent alone. Conclusions. Taken to gether these results suggest that although wortmannin increases the cytotox ic effficacy of some receptor-directed chimeras, potentiation may occur thr ough an alternative pathway not involving Pl 3-kinase inhibition.