Wortmannin, a phosphoinositide 3-kinase inhibitor, selectively enhances cytotoxicity of receptor-directed-toxin chimeras in vitro and in vivo
Davol, P.A.; Bizuneh, A.; Frackelton, A.R.
Anticancer Research 19(3a): 1705-1713
1999
ISSN/ISBN: 0250-7005 PMID: 10470104 Document Number: 505328
Background. Generalized resistance of some neoplastic cell lines to treatment with ligand-toxin chimeras has been attributed to an increased rate of lysosomal uptake and degradation following endocytosis of the chimera-receptor complex. Because phosphoinositide 3-kinase (Pl 3-kinase) activity is known to play a role in intracellular trafficking, particularly from endosomes to lysosomes, we hypothesized that co-exposing cells to the Pl 3-kinase inhibitor, wortmannin, might enhance cytotoxicity of ligand-toxin chimeras. Methods. In vitro, cytotoxicity of five receptor directed-toxin chimeras (bFGF-SAP, bFGF-PE, aFGF-PE, HBEGF-SAP, bFGF-gelonin) and an immunotoxin (11A8-SAP) was examined in the presence or absence of this Pl 3-kinase inhibitor against a panel of human neoplastic cell lines: SK-MEL-5 (melanoma), PA-1 (ovarian teratocarcinoma), DU145 (prostatic carcinoma) and MCF-7 (breast carcinoma). In vivo, antitumor activity of a treatment regimen combining wortmannin (1 or 2 mg/kg i.p.) and bFGF-SAP (10 mug/kg i.v.) once a week for 4 weeks was evaluated compared to administration of each agent alone in C3H/HeN mice implanted with the FSallC murine fibrosarcoma. Results. At concentrations greater than the reported Ki for Pl 3-kinase inhibition (1-10 muM), wortmannin enhanced cytotoxicity when combined with saporin or gelonin chimeras, but produced subadditive cytotoxicity when combined with Pseudomonas exotoxin chimeras. When low nanomolar concentrations selective for Pl 3-kinase inhibition (5-100 nM) were examined for effects on one receptor directed-toxin chimera, wortmannin dramatically enhanced bFGF-SAP cytotoxicity in three of the four cell lines. A different Pl 3-kinase inhibitor, LY294002 (Ki apprx 1 muM), however, failed to potentiate bFGF-SAP. When administered to mice, wortmannin combined with bFGF-SAP resulted in a significant decrease in tumor volumes compared to vehicle-treated controls that was not observed in mice treated with either agent alone. Conclusions. Taken together, these results suggest that although wortmannin increases the cytotoxic effficacy of some receptor-directed chimeras, potentiation may occur through an alternative pathway not involving Pl 3-kinase inhibition.