Suramin is an experimental anti-neoplastic agent which has shown promising activity against prostatic carcinoma and lymphoma in clinical trials. To elucidate its mechanism of action, suramin was examined for an effect on the transport of folate compounds by tumor cells. Influx of the anti-folate methotrexate via the reduced-folate carrier system of CCRF-CEM cells was found to be highly sensitive to inhibition by suramin but not to various other arylsulfonic acids. Inhibition by suramin was competitive, and the inhibition constant Ki was 1.3 microM, a value 3-fold lower than the Kt for half-maximal influx of methotrexate. Folate binding to the membrane-associated folate-binding protein of KB cells was not affected by suramin. Growth studies revealed that the response of human CCRF-CEM, KB, PC-3 and MCF-7 cells to methotrexate was antagonized from 6- to 17-fold by pharmacological levels (10-200 microM) of suramin. Conversely, growth inhibition was additive or synergistic when suramin was combined with metoprine, a lipophilic anti-folate which enters cells by diffusion. Synergism was observed between metoprine and suramin in CCRF-CEM cells, which take up folate exclusively through the reduced-folate carrier (inhibitable by suramin), whereas additivity was observed for KB cells, which rely largely on the folate-binding protein (unaffected by suramin) for folate import. Our results indicate that inhibition of cellular transport of folate compounds may explain part of the anti-neoplastic effects of suramin on tumor cells.

Purpose: RXDX-107 is a dodecanol alkyl ester of bendamustine encapsulated in human serum albumin (HSA) to form nanoparticles. The anti-tumor activity of bendamustine in solid tumor malignancies has been less impressive, partially due to short half-life. RXDX-107 was designed to extend the half-life and improve tissue biodistribution over bendamustine, which may result in superior efficacy and tolerability in patients with solid tumors. Experimental Design:The anti-tumor activity of RXDX-107 was measured in cellular anti-proliferation assay, cell-line derived xenograft (CDX) models and patient-derived xenograft (PDX) models. The mechanism of action and pharmacodynamics properties were measured by comet assay. The tumor accumulation was measured by a novel LC-MS/MS method. Results:In in vitro anti-proliferative studies, RXDX-107 displayed dose-dependent cytotoxicity against multiple solid tumor cell lines. While the IC50 of RXDX-107 were comparable to those of bendamustine, RXDX-107 displayed more complete cell killing. RXDX-107 exhibited enhanced pharmacodynamics properties, including stronger induction of pH2AX (a biomarker for DNA damage) and higher interstrand crosslinks (ICLs) formation. RXDX-107 significantly reduces tumor growth in human NSCLC xenograft models. RXDX-107 also showed single agent anti-tumor activities, including tumor regression in multiple PDX models of solid tumors including breast, lung, and ovarian cancer. Furthermore, the mode of action data exhibit slow and sustained release of bendamustine from RXDX-107, and high intratumoral accumulation of RXDX-107. Conclusions:Our preclinical data demonstrate potent and broad anti-tumor activity of RXDX-107 across a variety of solid tumor types, and support further clinical development of this novel drug candidate for the treatment of solid tumors.