Novel Miniaturized Drug Conjugate Leverages HSP90-driven Tumor Accumulation to Overcome PI3K Inhibitor Delivery Challenges to Solid Tumors.

The PI3K pathway is considered a master regulator for cancer due to its frequent activation, making it an attractive target for pharmacologic intervention. While substantial efforts have been made to develop drugs targeting PI3K signaling, few drugs have been able to achieve the inhibition necessary for effective tumor control at tolerated doses. HSP90 is a chaperone protein that is overexpressed and activated in many tumors and as a consequence, small-molecule ligands of HSP90 are preferentially retained in tumors up to 20 times longer than in normal tissue. We hypothesize that the generation of conjugates that use a HSP90-targeting ligand and a payload such as copanlisib, may open the narrow therapeutic window of this and other PI3K inhibitors. In support of this hypothesis, we have generated a HSP90-PI3K drug conjugate, T-2143 and utilizing xenograft models, demonstrate rapid and sustained tumor accumulation of the conjugate, deep pathway inhibition, and superior efficacy than the PI3K inhibitor on its own. Selective delivery of T-2143 and the masking of the inhibitor active site was also able to mitigate a potentially dose-limiting side effect of copanlisib, hyperglycemia. These data demonstrate that by leveraging the preferential accumulation of HSP90-targeting ligands in tumors, we can selectively deliver a PI3K inhibitor leading to efficacy in multiple tumor models without hyperglycemia in mice. These data highlight a novel drug delivery strategy that allows for the potential opening of a narrow therapeutic window through specific tumor delivery of anticancer payloads and reduction of toxicity.

Targeting the Somatostatin Receptor 2 with the Miniaturized Drug Conjugate, PEN-221: A Potent and Novel Therapeutic for the Treatment of Small Cell Lung Cancer.

Small cell lung cancer (SCLC) is an aggressive neuroendocrine carcinoma with a 95% mortality rate with no improvement to treatment in decades, and new therapies are desperately needed. PEN-221 is a miniaturized peptide-drug conjugate (∼2 kDa) designed to target SCLC via a Somatostatin Receptor 2 (SSTR2)-targeting ligand and to overcome the high proliferation rate characteristic of this disease by using the potent cytotoxic payload, DM1. SSTR2 is an ideal target for a drug conjugate, as it is overexpressed in SCLC with limited normal tissue expression. In vitro, PEN-221 treatment of SSTR2-positive cells resulted in PEN-221 internalization and receptor-dependent inhibition of cellular proliferation. In vivo, PEN-221 exhibited rapid accumulation in SSTR2-positive SCLC xenograft tumors with quick clearance from plasma. Tumor accumulation was sustained, resulting in durable pharmacodynamic changes throughout the tumor, as evidenced by increases in the mitotic marker of G2-M arrest, phosphohistone H3, and increases in the apoptotic marker, cleaved caspase-3. PEN-221 treatment resulted in significant antitumor activity, including complete regressions in SSTR2-positive SCLC xenograft mouse models. Treatment was effective using a variety of dosing schedules and at doses below the MTD, suggesting flexibility of dosing schedule and potential for a large therapeutic window in the clinic. The unique attributes of the miniaturized drug conjugate allowed for deep tumor penetration and limited plasma exposure that may enable long-term dosing, resulting in durable tumor control. Collectively, these data suggest potential for antitumor activity of PEN-221 in patients with SSTR2-positive SCLC.

Discovery of an SSTR2-Targeting Maytansinoid Conjugate (PEN-221) with Potent Activity in Vitro and in Vivo.

Somatostatin receptor 2 (SSTR2) is frequently overexpressed on several types of solid tumors, including neuroendocrine tumors and small-cell lung cancer. Peptide agonists of SSTR2 are rapidly internalized upon binding to the receptor and linking a toxic payload to an SSTR2 agonist is a potential method to kill SSTR2-expressing tumor cells. Herein, we describe our efforts towards an efficacious SSTR2-targeting cytotoxic conjugate; examination of different SSTR2-targeting ligands, conjugation sites, and payloads led to the discovery of 22 (PEN-221), a conjugate consisting of microtubule-targeting agent DM1 linked to the C-terminal side chain of Tyr3-octreotate. PEN-221 demonstrates in vitro activity which is both potent (IC50 = 10 nM) and receptor-dependent (IC50 shifts 90-fold upon receptor blockade). PEN-221 targets high levels of DM1 to SSTR2-expressing xenograft tumors, which has led to tumor regressions in several SSTR2-expressing xenograft mouse models. The safety and efficacy of PEN-221 is currently under evaluation in human clinical trials.

23814, an Inhibitory Antibody of Ligand-Mediated Notch1 Activation, Modulates Angiogenesis and Inhibits Tumor Growth without Gastrointestinal Toxicity.

Dysregulation of Notch signaling has been implicated in the development of many different types of cancer. Notch inhibitors are being tested in the clinic, but in most cases gastrointestinal and other toxicities have limited the dosage and, therefore, the effectiveness of these therapies. Herein, we describe the generation of a monoclonal antibody against the ligand-binding domain of the Notch1 receptor that specifically blocks ligand-induced activation. This antibody, 23814, recognizes both human and murine Notch1 with similar affinity, enabling examination of the effects on both tumor and host tissue in preclinical models. 23814 blocked Notch1 function in vivo, inhibited functional angiogenesis, and inhibited tumor growth without causing gastrointestinal toxicity. The lack of toxicity allowed for combination of 23814 and the VEGFR inhibitor tivozanib, resulting in significant growth inhibition of several VEGFR inhibitor-resistant tumor models. Analysis of the gene expression profiles of an extensive collection of murine breast tumors enabled the successful prediction of which tumors were most likely to respond to the combination of 23814 and tivozanib. Therefore, the use of a specific Notch1 antibody that does not induce significant toxicity may allow combination treatment with angiogenesis inhibitors or other targeted agents to achieve enhanced therapeutic benefit.

Total synthesis of 6-deoxypladienolide D and Assessment of Splicing Inhibitory Activity in a Mutant SF3B1 cancer cell line.

A total synthesis of the natural product 6-deoxypladienolide D (1) has been achieved. Two noteworthy attributes of the synthesis are (1) a late-stage allylic oxidation which proceeds with full chemo-, regio-, and diastereoselectivity and (2) the development of a scalable and cost-effective synthetic route to support drug discovery efforts. 6-Deoxypladienolide D (1) demonstrates potent growth inhibition in a mutant SF3B1 cancer cell line, high binding affinity to the SF3b complex, and inhibition of pre-mRNA splicing.