Ongoing Phase 1 study results of XMT-1001 announced by Mersana Therapeutics

Mersana Therapeutics, a platform-based cancer therapeutics company, announced today additional preliminary results from an ongoing Phase 1 study of its lead development candidate, XMT-1001, in patients with advanced solid tumors. The results were presented in a poster session at the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics in Boston, MA November 15-19, 2009. XMT-1001 is a conjugate of the broad-spectrum cytotoxic camptothecin (CPT) that employs Mersana's Fleximer® platform.

XMT-1001 has continued to demonstrate promising tumor activity and favorable pharmacokinetic and tolerability profiles in the Phase 1 trial. Forty nine patients with refractory solid tumors have received 149 cycles of XMT-1001 at dose levels ranging from 1.0-85 mg CPT equivalents/m2. Twelve of 46 evaluable patients with advanced, refractory tumors demonstrated evidence of at least six weeks of stable disease. Nine of the 12 patients had prolonged stable disease for at least 12 weeks, including two patients for nine months. In addition, pharmacokinetic data for XMT-1001 have demonstrated dose proportional increases in exposure to the drug and confirms the formation of its release products according to the compound's design. The side-effect profile of XMT-1001 has been predictable and no toxicities associated with non-Fleximer-linked CPT or irinotecan, such as hemorrhagic cystitis or severe diarrhea, have been observed. The maximum tolerated dose (MTD) has not been reached and the Phase 1 study continues to accrue patients.

"We are very pleased to see that the data in this ongoing Phase 1 study for XMT-1001 continues to support our goal of establishing proof of concept for the Fleximer platform," said Julie Olson, Chief Executive Officer of Mersana. "Since our last report at ASCO, we have reached higher dose levels, but the MTD for XMT-1001 has not yet been reached. We have enrolled substantially more subjects and continue to see prolonged stable disease and a favorable pharmacokinetic profile. We look forward to concluding the study once the MTD has been reached and continuing to optimize the dosing and treatment regimen for XMT-1001 in a Phase 2 trial next year."