BTK-PROTACs : New Hope for BTK Inhibitors Resistance

Bruton's tyrosine kinase (BTK) inhibitors have shown significant clinical efficacy, but an unmet clinical need remains due to acquired resistance. To address the problem of drug resistance, many strategies have been developed, such as developing non-covalent BTK inhibitors, allosteric modulators, multi-target inhibitors, and BTK-PROTACs. Today, we will primarily focus on BTK-PROTACs.

BTK Inhibitors

BTK is a non-receptor tec kinase found within the cytoplasm of hematopoietic cells. It is a key component of B cell receptor (BCR) signaling and is an important regulator of cell proliferation and cell survival in various B cell malignancies. Increased BTK levels are associated with cancer and autoimmune diseases. Abnormal BTK activity is closely linked to both cancer and autoimmune diseases. Insufficient or defective BTK activity can result in severe immunodeficiencies, like X-linked agammaglobulinemia. On the other hand, excessive BTK activity can cause abnormal B cell proliferation, which may lead to non-Hodgkin lymphoma and chronic lymphocytic leukemia.

BTK inhibitors can block the activation of BTK and its downstream signaling pathways, leading to inhibited B cell growth and cell death. Currently, the U.S. Food and Drug Administration (FDA) has approved four BTK inhibitors: ibrutinib (2013), acalabrutinib (2017), zanubrutinib (2020) and pirtobrutinib (2023). Tirabrutinib (2020) and orelabrutinib (2020) are two other BTK inhibitors that have been approved by the Japan Pharmaceuticals and Medical Devices Agency and the China Food and Drug Administration, respectively.

Structures-of-representative-BTK-inhibitors

Figure 1. Approved BTK inhibitors

BTK inhibitors have shown significant efficacy in treating B cell malignancies. However, with their widespread use, resistance has emerged. The primary mechanism of resistance is a mutation at the cysteine 481 position of BTK, preventing the inhibitors from covalently binding and inhibiting BTK. Additionally, mutations in the downstream PLCG2 are a common resistance mechanism. Together, these two types of mutations account for about 60%-70% of all BTK resistance cases. Other resistance mechanisms include point mutations like T474M and T316A, as well as bypass activation and compensatory activation of pathways such as PI3K and NFκB. Thus, novel therapeutic options are required to overcome the drug resistance issue.

btk-inhibitor-resistance

Figure 2. BTK Inhibitors Resistance

BTK-PROTACs To Overcome Drug Resistance

Proteolysis-targeting chimeras (PROTACs) are structurally composed of 3 elements: a warhead ligand that binds to the target protein, a ligand that binds to the E3 ubiquitin ligase, and a linker that couples the 2 ligands. PROTACs induce target protein degradation through the ubiquitin-proteasome system. This method helps avoid the need for high drug doses and reduces associated side effects. PROTACs only need to weakly bind to the target protein to "tag" it for degradation through the proteasome pathway. Even if BTK undergoes point mutations, PROTACs can still induce its degradation, making them a highly promising solution for overcoming BTK resistance.

In comparison to small molecule inhibitors, PROTACs offer significant advantages:

▶ Expands the druggable target space: PROTACs can target proteins that lack an active site.
▶ Catalytic mechanism of action: PROTACs only require catalytic amounts to function, avoiding the need for prolonged occupancy seen with small molecules to exert their effects.
▶ Overcomes resistance: PROTACs not only inhibit protein activity but also degrade the entire protein, disrupting both enzymatic and non-enzymatic functions (such as scaffolding or transcriptional roles), thereby overcoming resistance observed with traditional small molecules.

BTK-PROTACs In Clinical Trials

Based on these advantages, many companies are engaged in developing BTK PROTACs to overcome resistance issues associated with BTK inhibitors. BTK PROTACs promote the degradation of BTK protein, bypassing multiple resistance mechanisms and enhancing drug efficacy. This approach makes traditional BTK inhibitors more flexible and efficient in suppressing B cell growth, thereby improving the duration and effectiveness of treatment. This strategy holds significant potential in improving prognosis and survival rates for patients with B-cell malignancies.

BTK PROTACs remain a relatively new area of research. Currently, there are four BTK PROTAC molecules in clinical development, all in Phase I trials, including HSK29116 from Haisco Pharma, NX-2127 and NX-5948 from Nurix, and BGB-16673 by BeiGene.

BGB-16673

BGB-16673 is an orally available BTK-targeting chimeric degradation activation (BTK-CDAC) compound designed to degrade wildtype BTK and multiple mutant forms, including those that have developed resistance to BTK inhibitors in patients with disease progression. Preclinical models have shown that BGB-16673 can target and degrade BTK, overcoming C481S resistance, and hold promise for addressing the resistance issues associated with BTK inhibitors. BGB-16673 is currently being evaluated in phase 1 studies.

BGB-16673

Figure 3. BGB-16673: A BTK-Targeted CDAC [3]

An open-label Phase I study (BGB-16673-101) investigated the efficacy and safety of BGB-16673 in treating patients with relapsed/refractory chronic lymphocytic leukemia (R/R CLL) or small lymphocytic lymphoma (SLL). Enrolled R/R CLL/SLL patients had undergone at least two prior lines of therapy, had an ECOG performance status of 0-2, and had sufficient end-organ function. Patients must have previously received a covalent BTK inhibitor (cBTKi). BGB-16673 was dosed once daily orally in 28-day cycles. Dose escalation using a Bayesian optimal interval design with 6 dose levels (50-600 mg once daily) was planned.

As of 09 November 2023, 42 patients with CLL were enrolled (median age, 70 years [range, 50-91]) and 39 were treated (50 mg [n=1]; 100 mg [n=5]; 200 mg, [n=15]; 350 mg, [n=14]; 500 mg, [n=4]). For treated patients, the median number of prior therapies was 4 (range, 2-8).

The median follow-up time was 3.3 months (range, 0.1-16.7). One DLT occurred in 1 patient at 200 mg. MTD was not reached. The most common TEAEs were contusion (31%; no gr ≥3), fatigue (31%; no gr ≥3), diarrhea (26%; no gr ≥3), and neutropenia (23%; gr ≥3, 18%).

One patient (500 mg) had a TEAE of gr 3 hypertension. No atrial fibrillation was observed. Two patients had TEAEs that led to death (septic shock and pneumonia); neither was considered related to treatment. Two additional patients had TEAEs that led to treatment discontinuation (subdural hemorrhage and thyroid cancer). One patient had a dose reduction due to gr 2 arthralgia.

Thirty-five of 39 patients (90%) remain on therapy (4 discontinuations: progressive disease [n=1], AEs [n=3]). For 24 response-evaluable patients, the ORR was 67%, with all but 1 response ongoing. Responses were seen at the lowest dose, in patients previously treated with a cBTKi (n=16) and an ncBTKi (n=2), and in patients with and without BTK mutation.

BGB-16673-101

Figure 4. Treatment duration and response assessment in patients with CLL/SLL [3]

NX-2127

NX-2127 is an oral small molecule degrader of BTK and cereblon neosubstrates IKZF1 (Ikaros) and IKZF3 (Aiolos). It is comprised of a cereblon (CRBN)-binding moiety conjugated, via a linker, to a BTK-binding moiety. Compared to BTK inhibitors, NX-2127 may overcome tumor resistance associated with BTK inhibitor-induced resistance mutations.

NX-2127

Figure 5. Structure of NX-2127

In November 2023, the FDA placed the phase 1 trial (NCT04830137) of NX-2127, a first-in-class Bruton’s tyrosine kinase (BTK) inhibitor, on a partial clinical hold. IN March 2024, The FDA lifted the partial clinical hold on the phase 1 trial of NX-2127.

At the 2023 ASH conference, Nurix presented promising Phase I clinical trial data for NX-2127: in patients with relapsed or refractory (r/r) B cell malignancies, NX-2127 induced encouraging, rapid, and durable responses. Among 17 evaluable non-Hodgkin lymphoma (NHL) patients, there were 2 complete responses (CR) and 2 partial responses (PR). In 27 evaluable chronic lymphocytic leukemia (CLL) patients, 11 experienced PRs, with an overall response rate (ORR) of 40.7%.

Recently, Montoya and colleagues reported positive clinical results for NX-2127 in Science: NX-2127 achieved over 80% degradation of BTK, and 79% of treated evaluable CLL patients showed a clinical response, regardless of the BTK mutation type.

NX-2127-phase-1

Figure 6. Efficacy of NX-2127

NX-5948

NX-5948 is another BTK degrader designed by Nurix Therapeutics. Unlike NX-2127, NX-5948 lacks immunomodulatory activity and can cross the blood-brain barrier (BBB) in animal models. NX-5948 degrades both wild-type BTK and BTKi resistant mutants (C481) .

On June 16, 2024, Nurix Therapeutics announced positive results for NX-5948 in patients with R/R CLL at the European Hematology Association (EHA2024) Congress.

The data presented included safety results from all patients in the dose-escalation Phase 1a trial (n=79) and efficacy results for patients with relapsed or refractory CLL (n=31). Patients took NX-5948 orally once daily, with doses ranging from 50 mg to 600 mg. NX-5948 was well tolerated at all evaluated doses, with the most common treatment-related adverse events being purpura/bruising, thrombocytopenia, and neutropenia.

NX-5948 has demonstrated positive results from the ongoing Phase 1a clinical trial in patients with an objective response rate of 69.2% in heavily pretreated CLL patients including those with BTK inhibitor resistance mutations. With an emerging best-in-class profile, Nurix is expanding to Phase 1b in CLL with plans to initiate pivotal development in 2025.

NX-5948

Figure 7. NX-5948 Efficacy

HSK29116

HSK29116, developed by Haisco, is an oral small molecule BTK degrader. It selectively blocks BTK kinase activity and intervenes in B cell development by modulating signaling pathways, thereby controlling the progression of various B cell malignancies.

In preclinical studies, HSK29116 has overcome resistance issues caused by the C481S BTK mutation. Additionally, HSK29116 has demonstrated the ability to inhibit B cell proliferation without causing measurable loss of activity in other BTK family kinases. This kinase selectivity may help limit some off-target toxicities associated with currently available BTK inhibitors.

At the 2023 AACR conference, the Phase I study protocol for HSK29116 in patients with relapsed or refractory B cell malignancies was reported. Currently, an international multi-center Phase I clinical trial of HSK29116 for patients with relapsed/refractory B cell malignancies is underway.

Summary

BTK inhibitors have progressed through three generations but tend to develop resistance issues. To address this problem, many pharmaceutical companies are developing BTK PROTACs. However, most of these are still in the early stages of clinical development, and their future success remains to be seen.

The commonly used linkers in the development of PROTACs are PEGs. Biopharma PEG is a professional PEG derivatives supplier that provides multi functionalized PEG derivatives as PROTAC linkers. We have over 3000 high purity PEG linkers to empower our customers' PEGylation, bioconjugation, ADC, PROTAC drug development for pharmaceutical and biotech R&D.

References:
[1] Wen, T., Wang, J., Shi, Y. et al. Inhibitors targeting Bruton’s tyrosine kinase in cancers: drug development advances. Leukemia 35, 312–332 (2021).
[2] Sun SL, Wu SH, Kang JB et al. Medicinal Chemistry Strategies for the Development of Bruton's Tyrosine Kinase Inhibitors against Resistance. J Med Chem. 65(11):7415-7437. (2022)
[3] Preliminary efficacy and safety of the Bruton tyrosine kinase (BTK) degrader BGB16673 in patients with relapsed or refractory (R/R) CLL/SLL: Results from the phase 1 BGB-16673-101 study https://www.beigenemedical.com/CongressDocuments/Parrondo_BGB-16673-101_EHA_Abstract_2024.pdf
[4] Kinase-impaired BTK mutations are susceptible to clinical-stage BTK and IKZF1/3 degrader NX-2127 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11103405/
[5] EHA2024 Presentation. Retrieved June 17, 2024 from https://ir.nurixtx.com/static-files/e77e9b9c-2e5f-4409-b819-6a5bc02dce47