Acalabrutinib | ACP-196 | BTK Inhibitor | Cancer Drug I Treatment for Chronic Lymphocytic Leukemia

Acalabrutinib [(S)-4-(8-amino-3-(1-(but-2-ynoyl)pyrrolidin-2-yl)imidazo[1,5-a]pyrazin-1-yl)-N-(pyridin-2-yl)benzamide] is a novel, second generation, irreversible Bruton's tyrosine kinase (BTK) inhibitor that may show advantages in terms of binding specificity and drug-drug interactions compared to the first generation BTK inhibitor. It is being developed by Acerta Pharma BV for treatment of Chronic Lymphocytic Leukemia (CLL). The improved selectivity and target coverage of Acalabrutinib has been credited to its covalent bonding with a cysteine residue (Cys481) in the front position of the ATP-binding pocket [1,2].

Acalabrutinib: 2D and 3D Structure

Targeting Bruton's tyrosine kinase (BTK), an essential kinase in the B cell receptor (BCR) pathway in patients with chronic lymphocytic leukemia (CLL) has proven very effective. For the first generation BTK inhibitor Ibrutinib, clinical response rates greater than 70% and 75% and progression free survival greater than 2 years have been reported for previously treated patients. Survival and proliferation of CLL cells is highly dependent on microenvironment interaction, which must be taken into consideration when testing new drugs for CLL.

The activity of Acalabrutinib is as follows:

EC₅₀ (BTK enzyme activity) = less than 10 nM
EC₅₀ (LCK enzyme activity) = greater than 1 uM
EC₅₀ (SRC enzyme activity) = greater than 1 uM
EC₅₀ (FYN enzyme activity) = greater than 1 uM
EC₅₀ (LYN enzyme activity) = greater than 1 uM

Common Name: Acalabrutinib
Synonyms: ACP-196; ACP 196; ACP196
IUPAC Name: (S)-4-(8-amino-3-(1-(but-2-ynoyl)pyrrolidin-2-yl)imidazo[1,5-a]pyrazin-1-yl)-N-(pyridin-2-yl)benzamide
CAS Number: 1420477-60-6
SMILES:O=C(NC1=NC=CC=C1)C2=CC=C(C3=C4C(N)=NC=CN4C([C@H]5N(C(C#CC)=O)CCC5)=N3)C=C2
Mechanism of Action: Kinase Inhibitor; BTK Inhibitor
Indication: Chronic Lymphocytic Leukemia
Development Stage: Phase II
Company: Acerta Pharma BV

Acalabrutinib Synthesis

EP2734522A1: First reported synthesis.

Bruton's tyrosine kinase (Btk) is a nonreceptor enzyme in the Tec kinase family expressed among cells of hematopoietic origin including B cells, myeloid cells, mast cells and platelets, but not T cells, where it regulates multiple cellular processes. 

ACP-196 shows in vivo efficacy against human chronic lymphocytic leukemia cells xenografted to the NSG mouse model. Peripheral blood mononuclear cells from previously untreated CLL patients were injected intravenously into NSG mice. Mice received ACP-196 through the drinking water. The effect of ACP-196 on xenografted CLL cells from peripheral blood and spleen was assessed by flow cytometry. At all dose levels tested, ACP-196 significantly inhibited proliferation of human CLL cells in the spleens of NSG mice. As seen with other BCR inhibitors, ACP-196 transiently increased CLL cell counts in the peripheral blood in a dose dependent manner (p = 0.01). ACP-196 inhibited BCR signaling in vivo, as demonstrated by reduced phosphorylation of PLCy2 [1]. 

ACP-196 demonstrated higher selectivity for Btk when profiled against a panel of 395 non-mutant kinases (greater than 1 µM) in a competitive binding assay. IC₅₀ determinations on 9 kinases with a Cys in the same position as Btk showed ACP-196 to be the most selective. The improved selectivity is related to the reduced intrinsic reactivity of ACP-196’s electrophile. Importantly, unlike ibrutinib, ACP-196 did not inhibit EGFR, ITK or TXK. Phosphoflow assays on EGFR expressing cell lines confirmed ibrutinib’s EGFR inhibition (EC₅₀ = 47-66 nM) with no inhibition observed for ACP-196 at 10 µM. These data may explain the Ibrutinib-related incidence of diarrhea and rash [3].

For Canine Lovers 

ACP-196 has been tested in normal dogs and was found to be safe at doses to be used in this clinical trial. Given the demonstrated activity of ibrutinib in dogs with B cell lymphoma, it is expected that ACP-196 will show good activity in this setting as well.

Identifications:

1H NMR (Estimated) for Acalabrutinib

References:
1. Niemann, C. U.; et. al. Abstract 2624: The novel Bruton's tyrosine kinase inhibitor ACP-196 shows in vivo efficacy against human chronic lymphocytic leukemia cells xenografted to the NSG mouse model. Cancer Res 2014, 74, 2624.
2. Lannutti, B. J.; et. al. Abstract 408: ACP-196, an orally bioavailable covalent selective inhibitor of Btk, modulates the innate tumor microenvironment, exhibits antitumor efficacy and enhances gemcitabine activity in pancreatic cancer. Cancer Res 2015, 75, 408.
3. Covey, T.; et. al. Abstract 2596: ACP-196: a novel covalent Bruton's tyrosine kinase (Btk) inhibitor with improved selectivity and in vivo target coverage in chronic lymphocytic leukemia (CLL) patients. Cancer Res 2015, 75, 2596.
4. ClinicalTrials.gov ACP-196, a Novel Bruton Tyrosine Kinase (Btk) Inhibitor, for Treatment of Chronic Lymphocytic Leukemia. NCT02029443 (retrieved on 20-04-2015)
5. ClinicalTrials.gov An Open-label, Phase 1b Study of ACP 196 in Subjects With Waldenström Macroglobulinemia. NCT02180724 (retrieved on 20-04-2015)
6. Barf, T. A.; et. al. 4-imidazopyridazin-1-yl-benzamides and 4- imidazotriazin-1-yl-benzamides as btk-inhibitors. EP2734522A1