the vascular disrupting agent crolibulin binds to the colchicine binding site and produces antivascular and apoptotic effects. In a multisite phase 1 clinical study of crolibulin (NCT00423410), we measured treatment-induced changes in tumor perfusion and water diffusivity (ADC) using dynamic contrast-enhanced MRI (DCE-MRI) and diffusion-weighted MRI (DW-MRI), and computed correlates of crolibulin pharmacokinetics. 11 subjects with advanced solid tumors were imaged by MRI at baseline and 2-3 days post-crolibulin (13-24 mg/m 2). ADC maps were computed from DW-MRI. Pre-contrast T 1 maps were computed, co-registered with the DCE-MRI series, and maps of area-under-thegadolinium-concentration-curve-at-90 s (AUC 90s) and the Extended Tofts Model parameters k trans , v e , and v p were calculated. There was a strong correlation between higher plasma drug C max and a linear combination of (1) reduction in tumor fraction with AUC 90s > 15.8 mM s, and, (2) increase in tumor fraction with v e < 0.3. A higher plasma drug AUC was correlated with a linear combination of (1) increase in tumor fraction with ADC < 1.1 × 10 −3 mm 2 /s , and, (2) increase in tumor fraction with v e < 0.3. These findings are suggestive of cell swelling and decreased tumor perfusion 2-3 days post-treatment with crolibulin. The multivariable linear regression models reported here can inform crolibulin dosing in future clinical studies of crolibulin combined with cytotoxic or immune-oncology agents. Tumor vasculature differs fundamentally from normal blood vessels, presenting opportunities for selective targeting that have led to two main categories of therapeutics: antiangiogenic agents designed to prevent neovascularization, and Vascular Disrupting Agents (VDAs) that target endothelial cells and pericytes of established tumor vasculature and induce vascular collapse 1,2. Efforts in the former category have been more successful, with FDA approval being granted to bevacizumab, sunitinib, sorafenib, lenvatinib, and multiple other antiangiogenic agents. VDAs that have entered clinical testing as anti-cancer therapeutics include, combretastatin A4 phosphate 3 , ZD6126 4 , ombrabulin 5,6 , plinabulin 7 , and crolibulin 8,9. Clinical development of VDAs has been hampered by non-availability of effective biomarkers to identify an Optimal Biological Dose (OBD) rather than the Maximum Tolerated Dose (MTD) 10,11. The choice of companion diagnostic depends on the mode of drug action. For example, agents targeted to genetic alterations can be guided by assays of the specific molecular aberration or frequency of target presence in a given patient's tumor 12 , while nanoparticle drug penetration into solid tumors may be predicted by imaging biomarkers such as ferumoxytol-enhanced MRI 13 .
