Btm-3566, a Novel Activator of the Mitochondrial Stress Response Promotes Robust Therapeutic Responses in Vitro and In Vivo in Diffuse Large B-Cell Lymphoma

Relapsed/refractory diffuse large B-cell lymphomas (r/r-DLBCL) are a therapeutic challenge, especially in patients not suitable for high dose chemotherapy, stem cell transplantation or patients who fail CAR-T-cell therapy. r/r-DLBCLs are highly heterogeneous both clinically and molecularly, which imposes a pressing need to develop novel therapies to improve outcomes in patients independently of the molecular subtype. We describe here BTM-3566, a first-in-class compound with activity against a variety of B-cell malignancies but with greatest effect in DLBCL. BTM-3566 activates the mitochondrial integrated stress response (ISR) through a novel mechanism regulated by the mitochondrial protein FAM210B. BTM-3566 induces apoptosis in DLBCL lines in vitro and complete tumor regression in vivo in DLBCL PDX mouse models harboring genetic alterations associated with poor prognosis.

BTM-3566 is an oral small molecule based on a pyrazolothiazol-backbone, developed for treatment of diffuse large B-cell lymphoma (DLBCL). BTM-3566 induces apoptosis and complete cell killing in DLBCL lines a with an IC ₅₀ of ~200 - 500 nM. Responsive DLBCL cell lines include ABC, GCB, and double-hit and triple-hit lymphoma lines. Pharmacokinetic studies in mice showed suitability for once daily dosing, with > 50% of oral bioavailability and close to 6 hours of serum half-life. 14-day dosing in mice and dogs demonstrated excellent tolerability at therapeutic doses. BTM-3566 showed stability in human hepatocytes (IC < 5 ml/min*kg) as well and a favorable in vitro safety profile. In xenograft models using the double-hit DLBCL line SU-DHL-10, BTM-3566 treatment resulted in complete regression in all tumor-bearing animals. Most importantly, no subsequent tumor growth occurred for 2 weeks after cessation of therapy, indicating that treatment with BTM-3566 resulted in a durable complete remission in this model of double-hit DLBCL. Expansion studies into human DLBCL PDX models harboring a range of high-risk genomic alterations, including Myd88 mutations and MYC and BCL2 rearrangements, demonstrated response in 100% of the lines with complete tumor regression in 6 of 8 PDX models tested (Table 1).

Transcriptome and proteome analyses revealed that BTM-3566 strongly activated the ATF4-integrated stress response (ISR), indicated by phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) and subsequent upregulation of the transcription factor ATF4. Of the four eIF2a-kinases in the human genome we determined, using CRISPR-Cas 9 gene depletion, that HRI was uniquely required for BTM-3566 eIF2a phosphorylation, induction of ATF4 ISR and apoptosis. HRI is described as being activated by mitochondrial-related stress, including heme depletion, increased ROS generation or blockage of mitochondrial ATP synthesis which result in an increase in mitochondrial proteostasis including activation of mitochondrial protease OMA1. We determined that BTM-3566 activates OMA1 without acting as a classical mitochondrial toxin. Treatment with BTM-3566 induced OMA1-dependent OPA1 processing and mitochondrial fragmentation in as little as 30 minutes of treatment, in the absence of any reduction in mitochondrial oxygen consumption or membrane depolarization. This data indicates that BTM-3566 represents a new class of compounds that activate the mitochondrial protease OMA1.

Gene expression-based profiling of BTM-3566 sensitivity in over 400 cancer cell lines showed that FAM210B, a mitochondrial membrane protein, negatively correlated with response to BTM-3566. Notably, overexpression of FAM210B completely prevents OMA1 activation and causes complete resistance to BTM-3566-induced apoptosis in DLBCL cell line BJAB and the Burkitt lymphoma cell line Ramos. Thus, FAM210B serves as a strong predictor of BTM-3566 sensitivity, as well as revealing a novel mechanism of regulation of OMA1 activation.

In summary, we describe here a novel, highly potent activator of the mitochondrial ISR, which is well tolerated in mice and dogs, has favorable pharmacokinetics and induces robust DLBCL regression in-vivo. An IND application in B-cell malignancies will be completed by early Q1 2022 with initiation of first in human clinical trials the first half of 2022.

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