Mersana Therapeutics, Inc.

Good morning and welcome to the Mursana Therapeutics first quarter 2025 conference call. Currently, all participants are in a listen-only mode. There will be a question and answer session at the end of this call. I would now like to turn the call over to Mr. Jason Fredette, Senior Vice President, Investor Relations and Corporate Communications. Please proceed, sir.

Thank you, operator, and good morning, everyone. Before we begin, please note that this call will contain forward-looking statements within the meaning of federal securities laws. These statements may include, but are not limited to, those related to the potential clinical benefits of our product candidates and platforms, our clinical trial progress and design, addressable market opportunities, anticipated clinical milestones and data presentations, and cash runways. Each of these forward-looking statements is subject to risks and uncertainties that could cause actual results to differ materially from those projected in such statements. These risks and uncertainties are discussed in our annual report on Form 10-K filed with the Securities and Exchange Commission on March 3, 2025, and in subsequent SEC filings. Our filings are available at sec.gov and on our website, mursana.com. Except as required by law, we assume no obligation to update forward-looking statements publicly, even if new information becomes available in the future. On today's call, we have Marsana's Chief Executive Officer, Dr. Marty Huber, and our Chief Operating Officer and Chief Financial Officer, Brian DeSchuttner. With that, let me turn the call over to Marty to begin the discussion. Thank you, Jason, and good morning, everyone.

Let's begin by touching on last week's announcement of MRSANA's strategic restructuring and reprioritization plan. This plan includes several cost-savings initiatives. Among them are the reduction of about 55% of our workforce across functions, the elimination of our internal pipeline development efforts, a reduction of other research activities, and a narrowing of our clinical development work with Emily to focus on breast cancer. We will continue supporting Phase I dose escalation work for XMT 2056 and our ongoing collaborations our main objective however was to extend our cash runway into mid-2026 to give us the opportunity to generate important objective response rate and durability data for emily from both of our ongoing phase one dose extension cohorts many of our colleagues learned last week that they will be departing mersana today and others will be leaving in the near term i would like to take a moment to thank them for all they have contributed to our programs, and our patient-centric culture. Now let's move on to a very timely topic. Early this morning at the ESMO Breast Cancer 2025 Congress in Munich, Dr. Erica Hamilton, who heads up breast cancer research at the Sarah Cannon Research Institute, presented updated clinical data for EMILY, our dolosynthin B7H4 ADC. The presentation primarily focused on preliminary time-to-event data from patients with triple negative breast cancer, or TNBC, who are enrolled in our dose escalation and backfill cohorts. Safety and tolerability data in this population remained consistent with those previously reported, with no new safety signals. Now, before getting into the clinical activity data, it may be helpful to share a little context up front. First, it's worth noting that research has shown B7H4 expression is a negative prognostic factor in various cancers, including in TNBC. In other words, clinical outcomes in patients with higher B7H4 expression are generally worse than those for patients with lower expression. Additionally, it is helpful to know what the performance is for today's standard of care in late-line TNBC, namely single-agent chemotherapy. A key reference for this is ASCENT, Tridelby's registrational trial in relapsed and refractory TMDC. In that trial, patients receiving chemotherapy achieved an objective response rate, or ORR, of only 5%. The median progression-free survival, or PFS, was about 7 weeks, and median overall survival, or OS, was about 7 months. Those data were from patients who were naive to Topo1 ADCs. Ultimately, we believe that this is the type of time-to-event data that a new agent would need to beat in a potential randomized pivotal trial in post-Topo1 TMBC for full approval. And given these low bars, we believe such a randomized trial would not take much longer than a single-arm trial. With that, let's briefly recap the clinical activity data presented this morning. The presentation focused on our valuable patients with TMBC who received intermediate doses of EMILY, ranging from about 38 milligrams per meter squared up to about 67 milligrams per meter squared. Importantly, more than 80% of these TMBC patients had received a prior TOPA-180C. Among those patients with B7H4 low tumors who received four or fewer prior lines of treatment, the ORR was 0%. The median PFS was 6.4 weeks and the median OS was 5.7 months. But among those patients with what we have initially characterized as B7H4 high tumors who received four or fewer prior lines of therapy, the ORR was 29%. The median PFS was 16 weeks and the median OS had not yet been reached as of the data cutoff of March 8th. As a reminder, our current dose expansion cohorts are only enrolling TMBC patients who receive four or fewer prior lines of therapy, including at least one prior TOCO-180C. While some patients with B7H4 low tumor expression are being enrolled, our primary focus is on the B7H4 high TMBC population. And so, While the sample size from dose escalation and backfill is small, today's presentation sheds further light on why we continue to believe EMILY could represent a meaningful improvement over today's standard of care for patients with post-TOPO1 TMBC. The ESMO breast presentation also contained an update on clinical activity observed across all tumor types in dose escalation and backfill cohorts as of that data cutoff of March 8th. Eight of 26 evaluable patients with B7H4 high tumor expression who received intermediate doses of Emily achieved a confirmed response for an ORR of 31%. This is an increase from the 23% ORR that was reported based upon our December 2024 data cutoff. Further details are contained in the ESMO breast presentation, which can be accessed on the publication sections of our website at mursana.com. We will be sharing some additional clinical data from dose escalation and backfill cohorts across all tumor types based on that March 8 beta cutoff in an oral presentation at ASCO in a couple weeks. So where do we stand with our expansion work with Emily? Well, we're making great progress. Again, in expansion, we are focusing on patients with TNBC who have received one to four prior lines of therapy, including at least one prior DOPA1 ADC. Enrollment in our initial expansion cohort that is receiving 67.4 milligrams per meter squared dose of Emily every four weeks has advanced rapidly in 2025. As a reminder, we amended our clinical trial protocol in the first quarter of this year with the goal of mitigating proteinuria-related dose delays we had seen at higher doses of Emily. These proteinuria management guidelines have now been adopted at our clinical sites. I'm also happy to share that we recently initiated and have progressed patient enrollment in our second TNVC expansion cohort. These patients are receiving a starting dose of 44.5 milligrams per meter squared of Emily on days one and eight of the first four-week cycle, followed by 80 milligrams per meter squared every four weeks. We chose this regimen for a few reasons. First, All four of the evaluable B7H4 high patients who received the 44.5 mg per meter squared day one, day eight dose every four weeks in dose escalation and backfill cohorts achieved tumor reductions of at least 30%. Second, we believe our recent protocol amendment will enable us to maintain dose intensity and tolerability for our 80 mg per meter squared Q4 dose. And third, our PK work showed that exposures for this regimen are distinct versus our 67 mg per meter squared every four-week dose, which we believe may be helpful in the spirit of Project Optimus. As we continue advancing our work in expansion, we are also witnessing a series of developments that could significantly expand the post-TOCO1 patient pool. Up until now, in the breast cancer space, topo 180Cs have only been approved for relapsed and refractory patients. But in recent months, there have been multiple positive phase 3 readouts for topos in the frontline setting. Focusing specifically on the TMBC, as we've noted before, global TMBC revenues for Tredelvi in 2025 are projected to exceed $1 billion. Just a few weeks ago, positive top-line results were shared from Ascent4, a clinical trial for the combination of Tredelvi and Keytruda in frontline TNBC. This readout may enable Tredelvi to become the new standard of care for first-line PD-L1 positive TNBC. And with results from the Phase III Ascent III trial in PD-L1 negative patients also expected in the weeks ahead, we believe the post-TOPO1 TNBC patient population could expand substantially. In summary, we remain excited about Marsana's prospects, we're making great progress with expansion enrollment, and we are looking forward to sharing initial clinical data from expansion in the second half of this year. With that, let's turn the call over to Brian for our financial review.