Targeting angiogenesis with oral agents

Anti-Angiogenic/Vascular Effects of the mTOR Inhibitor Everolimus Are Not Detectable by FDG/FLT-PET

Noninvasive functional imaging of tumors can provide valuable early-response biomarkers, in particular, for targeted chemotherapy. Using various experimental tumor models, we have investigated the ability of positron emission tomography (PET) measurements of 2-deoxy-2-[(18)F]fluoro-glucose (FDG) and 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) to detect response to the allosteric mammalian target of rapamycin (mTOR) inhibitor everolimus. Tumor models were declared sensitive (murine melanoma B16/BL6 and human lung H596) or relatively insensitive (human colon HCT116 and cervical KB31), according to the IC(50) values (concentration inhibiting cell growth by 50%) for inhibition of proliferation in vitro (<10 nM and >1 microM, respectively). Everolimus strongly inhibited growth of the sensitive models in vivo but also significantly inhibited growth of the insensitive models, an effect attributable to its known anti-angiogenic/vascular properties. However, although tumor FDG and FLT uptake was significantly reduced in the sensitive models, it was not affected in the insensitive models, suggesting that endothelial-directed effects could not be detected by these PET tracers. Consistent with this hypothesis, in a well-vascularized orthotopic rat mammary tumor model, other antiangiogenic agents also failed to affect FDG uptake, despite inhibiting tumor growth. In contrast, the cytotoxic patupilone, a microtubule stabilizer, blocked tumor growth, and markedly reduced FDG uptake. These results suggest that FDG/FLT-PET may not be a suitable method for early markers of response to antiangiogenic agents and mTOR inhibitors in which anti-angiogenic/vascular effects predominate because the method could provide false-negative responses. These conclusions warrant clinical testing.

Vatalanib: the clinical development of a tyrosine kinase inhibitor of angiogenesis in solid tumours

The licensing of bevacizumab in patients with metastatic colorectal cancer has fueled research in angiogenesis. Vatalanib (PTK787/ZK 222584), a potent oral tyrosine kinase inhibitor with a selective range of molecular targets, has been extensively investigated and has shown promising results in patients with colorectal cancer in early trials. Dynamic contrast-enhanced MRI has been useful as a pharmacodynamic tool to define the dose that has a biological effect. The primary objectives of the Phase III CONFIRM (Colorectal Oral Novel Therapy for the Inhibition of Angiogenesis and Retarding of Metastases in First-line) studies were not met. However, an interesting pre-planned subset analysis in both studies showed that patients with high lactate dehydrogenase derived clinical benefit. Although this type of analysis should always be considered with caution, the Phase III clinical programme of vatalanib is continuing with further innovative studies looking at other indications and schedules for vatalanib.