Exhibit 99.1 Mersana Therapeutics Provides Business Update and Announces First Quarter 2024 Financial Results - Maximum tolerated dose not yet established in Phase 1 clinical trial of XMT-1660; enrollment in dose escalation and backfill cohorts continuing in parallel; expect to announce initial clinical data and initiate expansion in the second half of 2024 - Patient recruitment ongoing in Phase 1 clinical trial of XMT-2056; plan to advance dose escalation in 2024 - Continue to expect capital resources will ...