Surrogate biomarkers in evaluating response to anti-angiogenic agents: focus on sunitinib

Conventional methods to assess the clinical activity of new agents that target specific biological pathways involved in tumour pathology may not provide correlation with clinically relevant outcomes such as patient survival or progression-free disease, and new and alternative methods should be explored. Biomarkers can assist in evaluation, and once validated, serve as a surrogate for clinical activity. Angiogenesis, a process well known to be involved in tumour growth and metastasis, is the target of several agents available today in the treatment of cancer. Laboratory assays used to detect proteins involved in angiogenesis and emerging imaging approaches have provided the bulk of the biomarker data to date in this area, and have already corroborated aspects of the biochemical basis of anti-angiogenic strategy. This symposium article will provide a brief overview of biomarker data in several different tumour types and discuss the effect that sunitinib and other anti-angiogenic agents have on these biomarkers. Surrogate biomarkers discussed include soluble proteins found in the blood or urine, circulating endothelial cells and their progenitors, and non-invasive imaging techniques.

A preclinical review of sunitinib, a multitargeted receptor tyrosine kinase inhibitor with anti-angiogenic and antitumour activities

Sunitinib malate is an oral, multitargeted tyrosine kinase inhibitor that targets both angiogenic pathways (i.e., vascular endothelial growth factor receptor and platelet-derived growth factor receptor) and direct pro-oncogenic pathways (e.g., stem-cell factor receptor and FMS-like tyrosine kinase-3). Preclinical studies with this agent have indicated that it exhibits robust inhibitory activity against these targets. Clinical trial results have demonstrated the therapeutic potential of this agent and have implicated sunitinib targets in the pathophysiology of malignancies such as renal cell carcinoma and gastrointestinal stromal tumour. This paper reviews the preclinical data supporting the development of this agent and its translation from benchtop to bedside. It also highlights the importance of the multiple pathways that may be involved in cancer progression and the importance of these pathways in selected malignancies.

Blood-based biomarkers of SU11248 activity and clinical outcome in patients with metastatic imatinib-resistant gastrointestinal stromal tumor

PURPOSE

There is an unmet need for noninvasive markers to measure the biological effects of targeted agents, particularly those inhibiting the vascular endothelial growth factor (VEGF) receptor (VEGFR) pathway, and identify patients most likely to benefit from treatment. In this study, we investigated potential blood-based biomarkers for SU11248 (sunitinib malate), a multitargeted tyrosine kinase inhibitor, in patients with metastatic imatinib-refractory gastrointestinal stromal tumors.

EXPERIMENTAL DESIGN

Patients (n=73) enrolled in a phase I/II trial received SU11248 daily for 14 or 28 days followed by 14 days without treatment per cycle. Clinical benefit was defined as progression-free survival of >6 months. We assessed plasma markers, including VEGF and soluble VEGFR-2 (sVEGFR-2), and two cellular populations bearing VEGF receptors: monocytes and, in a subset of patients, mature circulating endothelial cells (CEC).

RESULTS

Compared to patients with progressive disease, patients with clinical benefit had significantly greater increases in CECs (0.52 versus -0.01 CEC/microL/d, P=0.03) and smaller decreases in monocyte levels (47% versus 60%, P=0.007) during cycle 1. VEGF increased by 2.2-fold and sVEGFR-2 decreased 25% during the first 2 weeks of treatment. Neither plasma marker correlated with clinical outcome although a modest inverse correlation was observed between sVEGFR-2 changes and plasma drug levels. Monocytes, VEGF, and sVEGFR-2 all rebounded towards baseline off treatment.

CONCLUSIONS

Monocytes, VEGF, and sVEGFR-2 were consistently modulated by treatment, suggesting that they may serve as pharmacodynamic markers for SU11248. Changes in CECs and monocytes, but not the plasma markers, differed between the patients with clinical benefit and those with progressive disease. These end points merit further investigation in future trials to determine their utility as markers of SU11248 activity and clinical benefit in gastrointestinal stromal tumors and other tumor types.