Bicycle Toxin Conjugates Showcase Early Activity and Tolerability in Urothelial, Ovarian Cancers

The bicycle toxin conjugates (BTCs) BT5528 and BT8009 have showcased preliminary signs of antitumor activity and encouraging tolerability when administered to patients with solid tumors such as urothelial cancer and ovarian cancer, according to findings from 2 early-phase trials.¹

Bicycles are fully synthetic short peptides controlled with small molecule scaffolds to form 2 loops that stabilize their structural geometry. Because this constraint enables target binding with high affinity and selectivity, this class of medicines are attractive candidates for drug development.

When BT5528, a second-generation BTC targeting EphA2, was given as a single agent at every-other-week doses ranging from 6.5 mg/m² to 10 mg/m² to 2 patients with urothelial cancer, both patients experienced tumor reductions, which translated to a partial response (PR) per RECIST v1.1 criteria. Moreover, among 5 patients with ovarian cancer and EphA2 positivity who received the agent at every-other-week doses ranging from 6.5 mg/m² to 8.5 mg/m², 80% (n = 4) experienced antitumor activity with the agent; this included 1 PR per RECIST criteria.

Among 11 response-evaluable patients with urothelial cancer who received the Nectin-4–targeting BTC BT8009 at either 2.5 mg/m² or 5.0 mg/m2, 36% achieved tumor reductions that constituted PRs under RECIST criteria, with reductions ranging from 37% to 89%.

Among 4 patients who received the agent at weekly dose of 2.5 mg/m², 3 had stable disease and 1 patient experienced a reduction of 37%, meeting the criteria of a PR. The disease control rate (DCR) in these patients was 75%. In 7 patients who received BT8009 at a weekly dose of 5.0 mg/m², 3 achieved stable disease, the DCR was 71%, and 43% experienced tumor reductions that met the criteria of a PR per RECIST criteria. Tumor reductions in this group ranged from 44% to 89%.

“We are pleased to provide a clinical update for 2 of our wholly-owned BTCs currently undergoing phase 1 dose-escalation trials in late-line cancer patients,” Dominic Smethurst, MA, MBChB, MRCP, MFPM, chief medical officer of Bicycle Therapeutics, stated in a press release. “We are delighted to see preliminary antitumor activity in both trials and across 2 tumor types, as well as to report tolerability profiles that may demonstrate differentiation from antibody-based approaches.”

BT5528

The BTC BT5528 targets EphA2, which is a member of the Ephrin superfamily of receptor tyrosine kinases; this regulates cell migration, adhesion, proliferation, and differentiation during development.² This target is overexpressed in many difficult-to-treat malignancies and its expression has been linked with disease progression and poor prognosis. This target has previously been explored in the development of antibody-drug conjugates but were found to come with significant safety concerns. BTCs such as BT5528 have been hypothesized to be able to overcome such concerns due to its short half-life and rapid kidney elimination.

BT5528 is comprised of a EphA2-targeting Bicycle, a valine-citrulline cleavable linker, and a cytotoxin MMAE payload. Notably, these EphA2 toxin conjugates have been shown to have preclinical target-dependent efficacy in cell- and patient-derived xenografts that were resistant to standard-of-care options.

In the ongoing phase 1/2 trial, a total of 24 patients were dosed with BT5528 before and after the use of a EphA2 immunohistochemistry assay. These patients had previously received a median of 7 lines of therapy.

Regarding safety, BT5528 was found to be well tolerated, with no signs of coagulopathy reported to date.

The BTC was dosed up to 8.5 mg/m² weekly and 10 mg/m² every other week. At a dose of 8.5 mg/m², some mild and transient neutropenia was reported but this was not considered to be a dose-limiting toxicity (DLT). When the every-other-week dose of 10 mg/m² was given, however, the following 2 DLTs were reported: grade 3 fatigue and grade 3 pneumonitis.

The most common grade 3 and higher effects that occurred with the BTC included neutropenia, anemia, and pneumonitis. Grade 5 tumor lysis syndrome and renal failure resulting from gastrointestinal-related dehydration were reported.

Based on these data and findings from preclinical studies, the recommended phase 2 dose for BT5528 is anticipated to fall in the range of 6.5 mg/m² and 8.5 mg/m² every other week. The agent will advance to exploration in expansion cohorts in 2022, where it will be examined in patients with urothelial and ovarian cancers, as well as a basket cohort of those with head and neck, non–small cell lung cancer, gastroesophageal, and triple-negative breast cancers.

The trial is slated to enroll up to 56 participants in the initial expansion cohorts, with the potential to further expand enrollment.

BT8009

The BTC BT8009 was designed to target Nectin-4, which is a cell adhesion molecule from the Nectin and Nectin-like family, that is known to be overexpressed in tumor cells and hypothesized to be an important player in tumor cell growth and proliferation; it had become a validated target for cytotoxin delivery.

BT8009 is comprised of the Nectin-4–targeting Bicycle, a valine-citrulline cleavable linker and a cytotoxin MMAE payload. The BTC was designed to be easily manufactured and to have a renal route of elimination that minimizes liver exposure. The drug is hypothesized to have efficacy in several Nectin-4–expressing cancer types such as breast cancer, pancreatic cancer, esophageal cancer, bladder cancer, and lung cancer.

As of September 30, 2021, 11 response-evaluable patients with urothelial cancer received treatment with single-agent BT8009 in the ongoing phase 1 portion of a phase 1/2 trial. Four of the patients received the agent at a weekly dose of 2.5 mg/m² and the remaining 7 received the agent at a weekly dose of 5.0 mg/m².

All patients included in the trial received at least 2 prior lines of therapy, with a median of 2 lines of treatment; number of previous therapies received ranged from 2 lines to 6 lines. At both doses examined, patients were found to have acceptable tolerability. Notably, no DLTs with BT8009 have been reported.

When given at the higher dose of 5.0 mg/m² weekly, the agent was estimated to administer over 35% more MMAE per 4-week dosing cycle than enfortumab vedotin (Padcev). Dose-escalation of the agent is ongoing, and participants are being enrolled to receive the agent at a weekly dose of 7.5 mg/m², as well as every-other-week dosing cohorts.

Next Steps With BTCs

“These data support our belief that the Bicycle platform offers a potentially differentiated approach to traditional toxin delivery,” Kevin Lee, PhD, chief executive officer of Bicycle Therapeutics, stated in a press release. “The data generated from these molecules provide a wealth of information and insights as we continue to expand the application of our technology and generate additional Bicycle-targeted therapeutics with the intention of making a meaningful difference to cancer patients. We look forward to providing additional clinical data on BT5528 and BT8009 next year and initiating our phase 1/2 study for BT7480 later this year.”

References

1. Bicycle Therapeutics announces interim BT5528 phase I clinical trial results and preliminary results from ongoing BT8009 phase I clinical trial. News release. Bicycle Therapeutics plc. October 7, 2021. Accessed October 19, 2021. https://bit.ly/3G0nC0K
2. Programs. Bicycle Therapeutics plc. Accessed October 19, 2021. https://bit.ly/3aQfx0j