Removal of Metabolic Liabilities Enables Development of Derivatives of Procaspase-Activating Compound 1 (PAC-1) with Improved Pharmacokinetics

Abstract

Procaspase-Activating Compound 1 (PAC-1) is an ortho-hydroxy-N-acylhydrazone that induces apoptosis in cancer cells by chelation of labile inhibitory zinc from procaspase-3. PAC-1 has been assessed in a wide variety of cell culture experiments and in vivo models of cancer, with promising results, and a Phase 1 clinical trial in cancer patients has been initiated (NCT02355535). For certain applications, however, the in vivo half-life of PAC-1 could be limiting. Thus, with the goal of developing a compound with enhanced metabolic stability, a series of PAC-1 analogues was designed containing modifications that systematically block sites of metabolic vulnerability. Evaluation of the library of compounds identified four potentially superior candidates with comparable anticancer activity in cell culture, enhanced metabolic stability in liver microsomes, and improved tolerability in mice. In head-to-head experiments with PAC-1, pharmacokinetic evaluation in mice demonstrated extended elimination half-lives and greater AUC values for each of the four compounds, suggesting them as promising candidates for further development.

Derivatives of Procaspase-Activating Compound 1 (PAC-1) and their Anticancer Activities

“…[15] PAC-1 and its derivatives induce apoptosis and are cytotoxic in cell culture to a diverse array of cancer cells, including cell lines derived from white blood cell cancers (lymphoma, [15, 42–51] leukemia, [15, 24, 44, 48–50, 52–55] and multiple myeloma [24, 55]), diverse carcinomas (breast, [15, 44, 48, 49, 52–54, 56–59] renal, [15] adrenal, [15, 60–62] colon, [15, 48, 55, 57–59, 63] lung, [15, 48, 49, 52–59, 63–67] cervical, [44, 55] gastric, [48, 49, 55, 57, 58, 63] ovarian, [55] liver, [48, 49, 55] prostate, [48, 49] and gallbladder [48, 49]), and other solid tumor types (melanoma, [15, 44, 48, 49] osteosarcoma, [55] neuroblastoma, [15, 55, 57, 58] and glioblastoma [48, 49, 68]). Patient-derived samples from colon cancer [15], chronic lymphocytic leukemia [23], and multiple myeloma [24] are also sensitive to PAC-1 and derivatives, and a therapeutic effect has been demonstrated in multiple murine tumor models [15, 48, 49, 56, 65, 66, 69] and in pet dogs with cancer.…”

“…Based on these results, a mechanism of action has been proposed for PAC-1 activity (Figure 3B): zinc binds loosely to procaspase-3 (dissociation constants of high nM to low μM), [81] inhibiting its activity. With a low- to mid-nM affinity for zinc, [42, 43, 50] PAC-1 efficiently competes with procaspase-3 for the loosely-bound zinc ions. This depletion of the labile zinc pool restores procaspase-3 enzymatic activity, allowing for cleavage of procaspase-3 to caspase-3 and the initiation of the execution pathway of apoptosis.…”

“…[23] A thorough discussion of the relationship between PAC-1 and other therapeutically relevant metal chelators was reported recently. [50] The considerable evidence suggesting that PAC-1 does not belong in the category of so-called Pan-Assay Interference Compounds (PAINS) [90] has also been extensively discussed. [50]…”

“…[50] The considerable evidence suggesting that PAC-1 does not belong in the category of so-called Pan-Assay Interference Compounds (PAINS) [90] has also been extensively discussed. [50]…”

“…[50] The library was constructed by the condensation of nine hydrazides ( 80a–i ) and five aldehydes ( 81a–e ). To minimize oxidative N -dealkylation, building blocks containing a benzoyl group ( 80c , 80e , 80g , and 80i ) were introduced.…”