Mersana Therapeutics, Inc.

Good morning and welcome to the Mursana Therapeutics' fourth quarter 2023 conference call and webcast. Currently, all participants are in 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 relating to our platforms, product candidates, business strategy, clinical trial execution and results, 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 10Q, filed with the Securities and Exchange Commission on November 7, 2023, 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 Descheidner. With that, let me turn the call over to Marty to begin our discussion.

Thank you, Jason, and good morning, everyone. It's great to be speaking with you again. Let's start today's call off with a brief description of our high-level aim here at Mursana. Although ADCs have firmly established a position at the forefront of oncology, there are significant platform and payload limitations that we believe are preventing this therapeutic class from realizing its full potential. At Mursana, we're focused on bringing forward innovations to address these limitations to meaningfully improve the efficacy and safety of ADCs. Our goals are first to minimize dose limiting platform toxicities. We believe the achievement of this goal could allow us to maximize the monotherapy potential of cytotoxic ADCs and also allow them to be used effectively in combination with other standard of care treatments. Something that simply isn't possible with many of today's ADCs. Second, we aim to avoid resistance mechanisms that appear to be hampering certain ADCs. And third, we're striving to extend the field well beyond cytotoxics and establish an entirely new class of ADC therapies that elicit a targeted innate immune response to combat cancer. With that as a backdrop, let's turn our attention to the progress we're making in accomplishing these objectives. And let's begin with our proprietary or statin payload that's being used in our next generation cytotoxic ADC platform, Dolacynthin. When we developed this payload, one of our core objectives was to avoid the dose-limiting neutropenia and peripheral neuropathy that is reported with ADCs based on the VC-MMAE platform and other first-generation ADC platforms. Our payload has controlled bystander effect, meaning that it initially is membrane permeable and capable of bystander killing. However, it has also been designed to be enzymatically converted to an active metabolite that is much less membrane permeable, resulting in its accumulation in the tumor and avoidance of off-target toxicity. While we view our payload as a core differentiator and advantage, the same can be said for the platform we're using to deliver that payload, dolusynthin. We have presented extensive preclinical data in the past, demonstrating important advantages for dolosynthin ADCs against ADCs produced using our own first-generation platform, doloflexin, and other platforms like BCMMAE. 2024 provides us with the opportunity to begin presenting the clinical data. Next week in Barcelona at the European Society of Gynecological Oncology, otherwise known as ESGO, clinical data will be presented for two discontinued product candidates, UPRI and XMT1592. Both of these candidates utilize the same NAPI2B antibody and the same proprietary payload with controlled bisphander effect. However, UPRI was developed using dolaflexin and 1592 was developed with dolasynthin. We believe these clinical data help to affirm that the severe neutropenia, peripheral neuropathy, and ocular toxicity that is frequently observed in trials of ADCs based on other platforms and payloads are uncommon with our payload. We also believe they clearly show that dolosynthin further reduces platform toxicities compared with doloflexin. Following these presentations in mid-2024, we plan to share our initial clinical data for XMT1660, our B7H4 targeting dolicentin ABC. We continue to be pleased with the progress we're making in our Phase I trial of validating the safety and tolerability of XMT1660 as a single agent in patients with solid tumors, including triple negative and estrogen receptor positive breast cancer, as well as ovarian and endometrial cancers. The dose escalation portion of the trial is ongoing. In fact, we just recently escalated to a dose of 59 milligrams per meter squared, which is the highest dose that we have investigated clinically with Adolescent and ADC. A maximum tolerated dose for XMT1660 still has not been established. In addition, the continuing escalating dose, we are also continuing to enroll patients in backfill cohorts to optimize dose and schedule. As is typical for phase one, we're enrolling a heavily pretreated patient population. Today, single agent chemotherapy is the standard of care for these types of patients, and their prognosis is exceedingly poor. For instance, the objective response rate in late stage triple negative breast cancer is estimated to be approximately 5% or less, with a duration of response that is less than four months. Today, most breast cancer patients here in the US are receiving Inher2 and Tredelvi early in their treatment. An increasing amount of data is emerging that shows patients are developing resistance following their first topo1 ADC treatment. These factors are presenting an urgent unmet need for new ADCs with alternative payloads that do not share these resistance mechanisms. We are enrolling many patients who have previously received at least one of these topo-ADCs in our phase one clinical trial. And we're looking forward to sharing initial data mid-year so we can begin to clinically characterize XMT1660's efficacy and safety profile. Now, while we're very excited about XMT1660 and dolosynthin, we believe IO may be the next significant frontier for ADCs. Our immunosynthin platform is designed to harness the power of sting and overcome the historic limitations of free systemic sting agonist and intratumoral injections. This platform has the potential to deliver a targeted and impactful one-two punch by activating sting in a target-dependent manner in tumor cells and in tumor-resident myeloid and dendritic cells, while also minimizing the risk of systemic exposure. XMT2056 is our lead immunosynthin ADC. We're currently in the process of restarting our phase one trial of this HER2-targeting ADC, following a lift of the clinical hold on this trial by the FDA in the fourth quarter of 2023. In phase one, we plan to enroll patients with a range of different HER2-positive tumors, including breast, gastric, colorectal, and non-small cell lung cancer, and we're looking forward to advancing dose escalation in 2024. In addition to our independent programs over the past two years, we also have entered into collaboration agreements with Johnson & Johnson, Merck KGA, and GSK. We remain very much engaged with these companies as we seek to maximize the potential of our ADC platforms and product candidates. So, in summary, Mursana entered 2024 with energy and excitement. We have two differentiated ADC platforms, platforms that we think could address significant limitations for today's ADCs. We also have two differentiated clinical stage assets, an upcoming data readout on XMT1660, and a strong balance sheet. On this latter point, let me turn the call over to our Chief Operating and Financial Officer, Brian Descheitner, to share more detail.