Combination therapy using molecular-targeted drugs modulates tumor microenvironment and impairs tumor growth in renal cell carcinoma

Tumor growth and metastasis are determined not by cancer cells alone but also by a variety of stromal cells, various populations of which overexpress platelet‐derived growth factor receptors (PDGF‐Rs). In addition, activation of PI3K‐AKT‐mammalian target of rapamycin (mTOR) signaling is frequently observed in many cancer types as well. mTOR comprises a serine/threonine kinase that increases the production of proteins that stimulate key cellular processes such as cell growth and proliferation, cell metabolism, and angiogenesis. In this study, we investigated the impact of molecular‐targeting agents including PDGF‐R and mTOR inhibitors on the tumor stroma of human kidney cancer and examined the efficacy of combination therapy with these agents against this disease. Treatment with sunitinib did not suppress tumor growth, but significantly decreased stromal reactivity, microvessel density, and pericyte coverage of tumor microvessels in an orthotopic mouse model. In contrast, treatment with everolimus decreased tumor growth and microvessel density but not stromal reactivity. However, sunitinib and everolimus in combination reduced both the growth rate and stromal reaction. These findings suggest that target molecule‐based inhibition of the cancer–stromal cell interaction appears promising as an effective antitumor therapy.

RET mutation and increased angiogenesis in medullary thyroid carcinomas

Advanced medullary thyroid cancers (MTCs) are now being treated with drugs that inhibit receptor tyrosine kinases, many of which involved in angiogenesis. Response rates vary widely, and toxic effects are common, so treatment should be reserved for MTCs likely to be responsive to these drugs. RET mutations are common in MTCs, but it is unclear how they influence the microvascularization of these tumors. We examined 45 MTCs with germ-line or somatic RET mutations (RETmut group) and 34 with wild-type RET (RETwt). Taqman Low-Density Arrays were used to assess proangiogenic gene expression. Immunohistochemistry was used to assess intratumoral, peritumoral and nontumoral expression levels of VEGFR1, R2, R3, PDGFRa, PDGFB and NOTCH3. We also assessed microvessel density (MVD) and lymphatic vessel density (LVD) based on CD31-positive and podoplanin-positive vessel counts, respectively, and vascular pericyte density based on staining for a-smooth muscle actin (a-SMA), a pericyte marker. Compared with RETwt tumors, RETmut tumors exhibited upregulated expression of proangiogenic genes (mRNA and protein), especially VEGFR1, PDGFB and NOTCH3. MVDs and LVDs were similar in the two groups. However, microvessels in RETmut tumors were more likely to be a-SMA positive, indicating enhanced coverage by pericytes, which play key roles in vessel sprouting, maturation and stabilization. These data suggest that angiogenesis in RETmut MTCs may be more intense and complete than that found in RETwt tumors, a feature that might increase their susceptibility to antiangiogenic therapy. Given their increased vascular pericyte density, RETmut MTCs might also benefit from combined or preliminary treatment with PDGF inhibitors.