PRMT5 Inhibitors Regulate DNA Damage Repair Pathways in Cancer Cells and Improve Response to PARP Inhibition and Chemotherapies.

Expression of protein arginine methyltransferase 5 (PRMT5) is highly positively correlated to DNA damage repair (DDR) and DNA replication pathway genes in many types of cancer cells, including ovarian and breast cancer. In the current study, we investigated whether pharmacologic inhibition of PRMT5 downregulates DDR/DNA replication pathway genes and sensitizes cancer cells to chemotherapy and PARP inhibition. Potent and selective PRMT5 inhibitors significantly downregulate expression of multiple DDR and DNA replication genes in cancer cells. Mechanistically, PRMT5 inhibition reduces the presence of PRMT5 and H4R3me2s on promoter regions of DDR genes such as BRCA1/2, RAD51, and ATM. PRMT5 inhibition also promotes global alternative splicing changes. Our data suggest that PRMT5 inhibition regulates expression of FANCA, PNKP, and ATM by promoting exon skipping and intron retention. Combining C220 or PRT543 with olaparib or chemotherapeutic agents such as cisplatin demonstrates a potent synergistic interaction in breast and ovarian cancer cells in vitro. Moreover, combination of PRT543 with olaparib effectively inhibits the growth of patient-derived breast and ovarian cancer xenografts. Furthermore, PRT543 treatment significantly inhibits growth of olaparib-resistant tumors in vivo. These studies reveal a novel mechanism of PRMT5 inhibition and suggest beneficial combinatorial effects with other therapies, particularly in patients with tumors that are resistant to therapies dependent on DNA damage as their mechanism of action. SIGNIFICANCE: Patients with advanced cancers frequently develop resistance to chemotherapy or PARP inhibitors mainly due to circumvention and/or restoration of the inactivated DDR pathway genes. We demonstrate that inhibition of PRMT5 significantly downregulates a broad range of the DDR and DNA replication pathway genes. PRMT5 inhibitors combined with chemotherapy or PARP inhibitors demonstrate synergistic suppression of cancer cell proliferation and growth in breast and ovarian tumor models, including PARP inhibitor-resistant tumors.

Evaluation of the MDR-MDCK cell line as a permeability screen for the blood-brain barrier.

The objectives of this study were to (1) characterize MDR-MDCK monolayers as an in vitro model to predict brain uptake potential; (2) examine the ability of MDR-MDCK monolayers to identify the brain uptake potential of compounds that interact with P-glycoprotein (P-gp). The study measured the bi-directional transport of 28 compounds across MDR-MDCK monolayers. The brain uptake of a subset of the compounds was determined in the rat brain perfusion model. Drug concentrations were analyzed by LC-MS-MS. CNS-positive drugs exhibited absorptive permeability coefficients (Papp, A-B) values ranging from 3.4 x 10(-6) to 20.2 x 10(-6) cm/s; whereas CNS-negative drugs showed Papp (A-B) ranging from 0.03 x 10(-6) to 0.83 x 10(-6) cm/s. Inhibition of P-gp by cyclosporin A (CsA) significantly reduced secretory flux of compounds known to be P-pg substrates, but only enhanced the absorptive flux of compounds with high efflux ratio (>100). In vitro results were confirmed by brain perfusion studies on selected compounds. MDR-MDCK monolayers can be used to classify compounds into CNS-positive or CNS-negative based on the permeability coefficients (Papp, A-B). Under our experimental conditions, compounds with Papp (A-B)>3 x 10(-6) cm/s have high brain uptake potential; compounds with Papp (A-B)<1 x 10(-6) cm/s are unable to penetrate the blood-brain barrier (BBB); the brain uptake of compounds with Papp (A-B)<1 x 10(-6) cm/s and a P-gp-mediated efflux ratio of >100 may be enhanced by inhibiting P-gp.

Cryopreserved human hepatocytes as alternative in vitro model for cytochrome p450 induction studies.

Induction of cytochrome P450 (CYP) by drugs is one of major concerns for drug-drug interactions. Thus, the assessment of CYP induction by novel compounds is a vital component in the drug discovery and development processes. Primary human hepatocytes are the preferred in vitro model for predicting CYP induction in vivo. However, their use is hampered by the erratic supply of human tissue and donor-to-donor variability. Although cryopreserved hepatocytes have been recommended for short-term applications in suspension, their use in studies on induction of enzyme activity has been limited because of poor attachment and response to enzyme inducers. In this study, we report culture conditions that allowed the attachment of cryopreserved human hepatocytes and responsiveness to CYP inducers. We evaluated the inducibility of CYP1A1/2 and CYP3A4 enzymes in cryopreserved hepatocytes from three human donors. Cryopreserved human hepatocytes were cultured in serum-free medium for 4 d. They exhibited normal morphology and measurable viability as evaluated by the reduction of tetrazolium salts (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt) by cellular dehydrogenases. Treatment with beta-naphthoflavone (10 microM) for 3 d increased ethoxyresorufin-O-deethylase activity (CYP1A1/2) by 6- to 11-fold over untreated cultures and increased CYP1A2 messenger ribonucleic acid (mRNA) expression by three- to eightfold. Similarly, treatment of cryopreserved human hepatocytes with rifampicin (25 microM) for 3 d increased testosterone 6 beta-hydroxylase activity (CYP3A4) by five- to eightfold over untreated cultures and increased CYP3A4 mRNA expression by four- to eightfold. The results suggest that cryopreserved human hepatocytes can be a suitable in vitro model for evaluating xenobiotics as inducers of CYP1A1/2 and CYP3A4 enzymes.