(M1130-02-11) CNS Delivery of Novel DNA Damage Response (DDR) Inhibitors for the Treatment of Brain Tumors

Purpose: Glioblastoma (GBM) is an aggressive and highly infiltrative primary brain tumor. Standard of care for GBM involves maximal surgical resection followed by radiation and concurrent/maintenance chemotherapy that kills tumor cells by inducing DNA damage. DNA damage response pathways present a major resistance to effective brain tumor therapy. Therefore, inhibiting the key enzymes regulating the DNA damage response, namely ataxia-telangiectasia mutated kinase (ATM) and ataxia telangiectasia and Rad3-related protein (ATR) would lead to effective radio- and chemo-sensitization resulting in enhanced cytotoxicity. Numerous DDR inhibitors are in pre-clinical and clinical phases of development. The clinical efficacy of these DDR inhibitors will depend on their ability to cross the blood brain barrier (BBB) and attain therapeutic levels at the target site. The purpose of the current study is to determine the key pharmacokinetic (PK) parameters and mechanisms that influence the central nervous system (CNS) delivery of two novel DDR inhibitors – WSD0628, a potent ATM inhibitor and BAY1895344, a potent ATR inhibitor.

Methods: Pharmacokinetic and CNS distribution studies for both drugs were individually conducted in FVB wild-type (WT) mice following intravenous (IV) and oral (PO) dosing (5 mg/kg WSD0628 and 20 mg/kg BAY1895344). Plasma, brain, and spinal cord samples were harvested at pre-determined time points after dosing and the drug concentrations were measured using LC-MS/MS. PK parameters, namely systemic clearance (CL), volume of distribution (V_d), half-life (t_1/2), area under the curve (AUC) were calculated using Phoenix WinNonlin (v 8.3, Certara USA Inc., Princeton, NJ). The partition coefficient of total drug (K_p) in brain and spinal cord was determined as the ratio of the exposure (AUC) in the tissue of interest to the exposure in plasma. The free fraction of the drugs was measured in plasma, brain and spinal cord using rapid equilibrium dialysis (RED). The unbound partition co-efficient (Kp,_uu) was determined by correcting the Kp with the ratio of free fraction in the tissue of interest to free fraction in plasma.

Results: Our results indicate that WSD0628 has a half-life of 4.11 hrs in mice. It has a high oral bioavailability (0.95). The total partition coefficient in brain (Kp, brain) is 0.05 to 0.11 after IV and PO dosing, respectively. The free fraction in plasma is 2.01 + 0.02 % and in brain is 1.22 + 0.40%. The unbound brain partition coefficient (Kp,_uu brain) is 0.03 and 0.06 after IV and PO dosing, respectively. BAY1895344 has a short half-life (1.2 hrs) and lower oral bioavailability (0.49) in mice. It has a higher total brain partition coefficient as compared to WSD0628 (0.32 and 0.58 after IV and PO dosing respectively). It has a higher free fraction in plasma (8.02 + 0.88%), however a very low free fraction in brain (0.73 + 0.09%). Thus, the unbound partition coefficient in the brain is similar to WSD0628 (0.03 and 0.05 after IV and PO dosing, respectively). Based on the concentration ratios over time, WSD0628 and BAY1895344 attain rapid equilibrium in the CNS and the concentration ratios stay constant thereafter.

Conclusion: Based on the results, WSD0628 and BAY1895344 are highly bound to plasma components and CNS tissues. This leads to a low Kp,_uu value, a key parameter to determine the brain penetrability of drugs. However, the drugs are highly potent inhibitors of ATM and ATR; the key regulators of the DDR pathways. Concentrations attained in brain after a single dose of 5 mg/kg of WSD0628 are sufficient to engage with the target over the desired time frame and elicit a response after DNA damage by radiotherapy, hence this drug shows promising potential and further evaluations are underway to determine if its brain distribution is limited by efflux transporters at the BBB. Meanwhile, evaluations are underway for BAY1895344 to evaluate the mechanisms of CNS delivery and desired concentration at the site of action for effective chemo-sensitization. Combining the knowledge of pharmacokinetics of the drugs and the kinetics of the targeted biological pathways will help to predict and develop effective dosage regimen for clinical trials.

Acknowledgments: Funding sources: U19CA264362-01; 5U01CA227954-05; R01HD099543-03
The authors declare no conflict of interest.

WSD0628 Concentration time profiles after 5 mg/kg dose of WSD0628 (a) IV (b) PO; Total partition coefficient (brain: plasma, spinal cord: plasma) after 5 mg/kg dose of WSD0628 (c) IV (d) PO



BAY1895344 Concentration time profiles after 20 mg/kg dose of BAY1895344 (a) IV (b) PO; Total partition coefficient (brain: plasma, spinal cord: plasma) after 5 mg/kg dose of BAY1895344 (c) IV (d) PO