SPARC overexpression enhances sensitivity to nab-paclitaxel in vivo

3480 Background: SPARC (Secreted Protein Acidic and Rich in Cysteine) is overexpressed in many cancers including breast, prostate, lung, brain, head-and-neck, and kidney. We have previously shown that SPARC expression appeared to correlate with response to nanoparticle albumin-bound (nab) paclitaxel (Abraxane, ABX) in head-and-neck cancer patients, and increase in SPARC suggested improved response to ABX in tumor models. To further define the role of SPARC, a cell line engineered to overexpress SPARC was used to test whether SPARC overexpression would result in increased albumin binding and increased response to ABX in vivo. Methods: Recombinant human SPARC was expressed and purified using HEK 293 cells. Biologic activities of rhSPARC were characterized by testing cytotoxicity against human cancer cell lines and normal rat hepatocytes, angiogenesis in a HUVEC tube formation assay, and affinity for albumin in a solid phase binding assay with Alexa 488 labeled BSA binding to rhSPARC immobilized onto PVDF. To examine the effect of SPARC overexpression on tumor response to ABX, SPARC cDNA was subcloned into an expression vector and stable SPARC overexpressing lines of PC3 prostate cancer cells were generated by calcium phosphate transfection and G418 selection. The PC3 SPARC overexpressor (HN104) and WT PC3 were grown as xenografts in vivo and examined for response to ABX (30 mg/kg, qdx5, N=5). Results: Purified rhSPARC had a N-terminal sequence identical to native human SPARC, expected amino acid composition, and mature, low mannose glycosylation. In the HUVEC tube formation assay, rhSPARC was pro-angiogenic at 10 µg/mL and anti-angiogenic at 100 µg/mL. rhSPARC was noncytotoxic against HT29, MX-1, HepG2, and normal rat hepatocytes at all dose levels. The SPARC albumin binding assay revealed a pattern of saturable and specific binding with an estimated Kd of 700 µM. The HN104 SPARC overexpressing xenograft exhibited enhanced response to ABX with T-C of 36 days compared with the parent WT PC3 xenograft with T-C of 25 days (p Conclusions: rhSPARC exhibited an affinity for albumin at physiologic concentrations, potentially leading to targeting of nanoparticle albumin-bound drugs in vivo. Exogenous rhSPARC stimulated angiogenesis (tube formation) in vitro, supporting the role of SPARC in tumor angiogenesis and invasiveness. The SPARC overexpressing prostate cancer PC3 xenograft exhibited enhanced response to ABX relative to WT PC3, supporting the hypothesis that SPARC overexpression could lead to enhanced anti-tumor effect of nanoparticle albumin-bound drugs.