A synthetically lethal nanomedicine delivering novel inhibitors of polynucleotide kinase 3'-phosphatase (PNKP) for targeted therapy of PTEN-deficient colorectal cancer.

Phosphatase and TENsin homolog deleted on chromosome 10 (PTEN) is a major tumor-suppressor protein that is lost in up to 75% of aggressive colorectal cancers (CRC). The co-depletion of PTEN and a DNA repair protein, polynucleotide kinase 3'-phosphatase (PNKP), has been shown to lead to synthetic lethality in several cancer types including CRC. This finding inspired the development of novel PNKP inhibitors as potential new drugs against PTEN-deficient CRC. Here, we report on the in vitro and in vivo evaluation of a nano-encapsulated potent, but poorly water-soluble lead PNKP inhibitor, A83B4C63, as a new targeted therapeutic for PTEN-deficient CRC. Our data confirmed the binding of A83B4C63, as free or nanoparticle (NP) formulation, to intracellular PNKP using the cellular thermal shift assay (CETSA), in vitro and in vivo. Dose escalating toxicity studies in healthy CD-1 mice, based on measurement of animal weight changes and biochemical blood analysis, revealed the safety of both free and nano-encapsulated A83B4C63, at assessed doses of ≤50 mg/kg. Nano-carriers of A83B4C63 effectively inhibited the growth of HCT116/PTEN-/- xenografts in NIH-III nude mice following intravenous (IV) administration, but not that of wild-type HCT116/PTEN+/+ xenografts. This was in contrast to IV administration of A83B4C63 solubilized with the aid of Cremophor EL: Ethanol (CE), which led to similar tumor growth to that of formulation excipients (NP or CE without drug) or 5% dextrose. This observation was attributed to the higher levels of A83B4C63 delivered to tumor tissue by its NP formulation. Our data provide evidence for the success of NPs of A83B4C63, as novel synthetically lethal nano-therapeutics in the treatment of PTEN-deficient CRC. This research also highlights the potential of successful application of nanomedicine in the drug development process.

Nano-Delivery of a Novel Inhibitor of Polynucleotide Kinase/Phosphatase (PNKP) for Targeted Sensitization of Colorectal Cancer to Radiation-Induced DNA Damage.

Inhibition of the DNA repair enzyme polynucleotide kinase/phosphatase (PNKP) increases the sensitivity of cancer cells to DNA damage by ionizing radiation (IR). We have developed a novel inhibitor of PNKP, i.e., A83B4C63, as a potential radio-sensitizer for the treatment of solid tumors. Systemic delivery of A83B4C63, however, may sensitize both cancer and normal cells to DNA damaging therapeutics. Preferential delivery of A83B4C63 to solid tumors by nanoparticles (NP) was proposed to reduce potential side effects of this PNKP inhibitor to normal tissue, particularly when combined with DNA damaging therapies. Here, we investigated the radio-sensitizing activity of A83B4C63 encapsulated in NPs (NP/A83) based on methoxy poly(ethylene oxide)-b-poly(α-benzyl carboxylate-ε-caprolactone) (mPEO-b-PBCL) or solubilized with the aid of Cremophor EL: Ethanol (CE/A83) in human HCT116 colorectal cancer (CRC) models. Levels of γ-H2AX were measured and the biodistribution of CE/A83 and NP/A83 administered intravenously was determined in subcutaneous HCT116 CRC xenografts. The radio-sensitization effect of A83B4C63 was measured following fractionated tumor irradiation using an image-guided Small Animal Radiation Research Platform (SARRP), with 24 h pre-administration of CE/A83 and NP/A83 to Luc+/HCT116 bearing mice. Therapeutic effects were analyzed by monitoring tumor growth and functional imaging using Positron Emission Tomography (PET) and [18F]-fluoro-3'-deoxy-3'-L:-fluorothymidine ([18F]FLT) as a radiotracer for cell proliferation. The results showed an increased persistence of DNA damage in cells treated with a combination of CE/A83 or NP/A83 and IR compared to those only exposed to IR. Significantly higher tumor growth delay in mice treated with a combination of IR and NP/A83 than those treated with IR plus CE/A83 was observed. [18F]FLT PET displayed significant functional changes for tumor proliferation for the drug-loaded NP. This observation was attributed to the higher A83B4C63 levels in the tumors for NP/A83-treated mice compared to those treated with CE/A83. Overall, the results demonstrated a potential for A83B4C63-loaded NP as a novel radio-sensitizer for the treatment of CRC.