Inclusion Criteria:

- Males and females with histologically confirmed metastatic colorectal cancer

- Measurable disease according to Response Criteria for Solid Tumors (RECIST)

- At least 1 prior therapeutic regimen (chemotherapy or biologic)

- ECOG performance status of 0 or 1

- Life expectancy ≥3 months

- Adequate organ and marrow function

- No other malignancies within 5 years

- Signed informed consent

Exclusion Criteria:

- Radiation to ≥25% of bone marrow within 30 days of XL999 treatment

- Treatment with systemic anticancer therapy within 30 days of XL999 treatment

- Subject has not recovered to ≤ grade 1 or to within 10% of baseline from adverse
events due to other medications administered >30 days prior to study enrollment

- History of or known brain metastases, current spinal cord compression, or
carcinomatous meningitis

- Uncontrolled and/or intercurrent illness

- Pregnant or breastfeeding females

- Known HIV