Mersana Therapeutics, Inc.

Good morning and welcome to Mursana Therapeutics' second quarter 2024 conference call and webcast. Currently, all participants are in a listen-only mode. There will be a question and answer session at the end of this call. Please note, this call is being recorded. I would now like to turn the call over to Jason Fredette, Senior Vice President, Investor Relations and Corporate Communications.

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 our platforms, product candidates, business strategy, clinical trial execution and data, business development efforts, and cash runway. 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 quarterly report on Form 10-Q filed with the Securities and Exchange Commission on May 9, 2024, 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 Mursana's President and Chief Executive Officer, Dr. Marty Huber, and our Chief Operating Officer and Chief Financial Officer, Brian Descheitner. With that, let me turn the call over to Marty to begin our discussion.

Thank you, Jason, and good morning, everyone. The second quarter of 2024 was a time of continued progress at Mursana as we advanced dose escalation in Phase I clinical trials of XMT1660, our lead dolosynthin ADC candidate, and XMT2056, our lead immunosynthin ADC candidate. At the same time, we made further progress in our collaborations while also benefiting from our efforts to reduce our operating expenses last year. We believe these collective accomplishments have put us in a strong position as we approach our initial clinical data readout for XMT1660, which is planned for the second half of this year. Let's begin with that program. XMT1660 is an ADC we developed using Dolasynthin, our next generation cytotoxic ADC platform. This candidate targets B7H4, a cell surface protein within the B7 family that can suppress anti-tumor immunity and can serve as a negative prognostic indicator for multiple tumor types. In the dose escalation portion of our ongoing Phase I clinical trial, we are enrolling patients with tumor types that most commonly express high levels of B7H4. These include patients with recurrent triple negative and hormone receptor positive breast cancers, as well as endometrial and ovarian cancers, all areas with high unmet medical need. For instance, as many know, Tridelvi and or Inher2 are being used to treat the vast majority of recurrent triple negative breast cancer patients in the U.S. today. Emerging clinical data continue to suggest that patients can develop resistance to ADCs with topo1 inhibitor payloads. As a result, we are hearing an increasing call among treating physicians for new ADCs with alternative payloads that aren't subject to topo1 or PGP efflux resistance mechanisms. XMT1660 fits this profile, and we are enrolling many patients who have received a prior topo1 ADC in our trial. Dose escalation remains ongoing and we still have not established a maximum tolerated dose. In fact, we are currently at a dose of 80 milligrams per meter squared in escalation. This is well beyond the dose levels we were able to reach clinically with any of our prior ADCs. We believe our ability to continue to dose escalate can be attributed to two factors. The first is dolasynthin's ability to reduce toxicities commonly associated with other ADC platforms like neutropenia, neuropathy, and ocular toxicity, as well as those that were seen with our first-generation platform, dolaflexin. And the second factor is that, based on data that has been reported to date, there does not appear to be an obvious on-target liability with B7H4. In parallel with our dose escalation work, which includes the enrollment of backfill cohorts, we are also proactively exploring different dosing schedules with XMT1660 with the aim to optimize efficacy and safety. This is included every four weeks, as well as more frequent dosing regimens. At the same time, we also are progressing our biomarker strategy in preparation for expansion and potential later stages of development. All this work is aimed at building a robust data set that can inform important strategic decisions as we seek to position XMT1660 as a potential best-in-class asset, one that we believe may have the opportunity to serve both as a monotherapy and in combination with standards of care that may be inaccessible for the other B7H4 ADCs in development. We continue to expect that we will be in a position to announce our initial clinical data and initiate expansion in the second half of this year. This readout will include safety, tolerability, efficacy, and biomarker data. Now, let's turn to Immunosynthin and XMT2056. Immunosynthin is an innate immune-stimulating ADC platform. Last month, we were pleased to publish preclinical data and nature communications regarding some of the key mechanistic underpinnings of our approach to activate sting in a targeted manner using an ADC. This included in vitro and in vivo data demonstrating a one-two punch consisting of target-dependent sting activation in both tumor cells and tumor-resonant immune cells. The data also showed increased anti-tumor efficacy and reduced serum cytokine elevations in comparison to a free sting agonist. XMT2056 is our lead sting agonist ADC candidate that we developed using immunosynthin. It targets a novel HER2 epitope that we believe could enable it to not only be an effective monotherapy, but also could allow for eventual combinations with a range of other agents, including other HER2 targeted therapies. The dose escalation portion of our phase one trial is advancing and is enrolling patients with HER2 positive tumors, including breast, gastric, colorectal, and non-small cell lung cancer. We expect to make good progress in escalation this year. And finally, I'm pleased to report that we continue to advance our discovery collaborations with Johnson & Johnson and Merck KGA. In fact, we have been performing CMC activities to support J&J, and earlier this month, we earned another milestone, this one for $8 million, related to that Dolacenthen collaboration. Payment of this milestone is due in the third quarter of 2024. Now, let's turn things over to Brian for our Q2 financial update.