Improving Efficiency of Radium-223 via Pharmacological Blockade of Gastrointestinal Uptake and Increase Bone targeting

Objectives: ²²³RaCl₂ is the first approved alpha-emitting therapy utilized for the treatment of castrate resistant prostate cancer and extends overall survival of patients with bone metastatic disease (1). While well-tolerated, significant gastrointestinal (GI) accumulation of ²²³Ra occurs early after administration (within minutes), associated with lasting radiotoxicities due to the slowexcretion of ²²³Ra and daughters (2). Additionally, more than half ²²³Ra injected is excreted, resulting in a direct loss of drug and decrease in therapeutic efficacy. For safer and more effective use of ²²³Ra, we hypothesized that due to its rapid accumulation in the GI, uptake in enterocytes is mediated by ion channels and that blocking this pathway may prevent radiotoxicity and increase potency. Here, we investigate ion channel modulators combined with ²²³RaCl_2, in vitro, using human organoids and, in vivo, in animal model of disease to demonstrate improved pharmacokinetics. Methods: ²²³Ra transfer through GI organoids was examined under the influence of a library of ion channel modulators utilizing primary human duodenal enteroids grown on polymeric trans-well filters (Fig. 1A/B/C). Radium migration was measured using light scintillation counting. Membrane integrities were checked post drug incubation and radioactive migration. Lead compounds from the ion channel screen were determined by magnitude of blocking or activating ²²³Ra transport relative to control. We then evaluated the effect of the drugs in vivo combined with ²²³RaCl₂ in skeletally mature mice. ²²³Ra organ distribution was assessed under varying time points (15 min to 240 h p.i.; Fig.1E/F). Toxicology was conducted over 20 days, monitoring animal weights in pair with blood chemistry analysis. Kidneys were harvested post-mortem and submitted to histopathology. Results: Amiloride, a NaH/K⁺ channel blocker, and NS-1619, K⁺ channel activator, presented significant effects in ²²³Ra membranar transport, where in vitro Amiloride reduced by 0.7 fold the radioisotopic transfer and NS-1619 increased by 2-fold ²²³Ra pumping though monolayers as compared to non-treated wells. The in vivo evaluation of selected ion modulators in combination with ²²³RaCl₂ demonstrated a 2-fold increase in stomach uptake of ²²³Ra for NS-1619 and a significantly lower stomach uptake for Amiloride (P<0.05) as compared to no combination (Fig.1D). More importantly, ²²³Ra bone uptake was observed to double in presence of Amiloride at 24 and 48 h p.i. and then stabilize to control level over 10 days (Fig. 1E/F). Animals treated with the combination Amiloride + ²²³RaCl₂ showed an increase in weight similar to untreated cohort; no renal tissue damages were seen following histopathologic evaluation. Blood chemistry reports no noticeable differences in enzymes or protein levels comparing ²²³RaCl₂ with combination or Amiloride alone. Only a higher K⁺ blood retention was seen at 7 days post-treatment for the combination cohort. The combination Amiloride/²²³RaCl₂ resulted in: no noted toxicity, significantly increased ²²³Ra uptake in the bone while partially neutralizing ²²³Ra compartmentalization in the GI tract. Organ-level dosimetric analyses confirmed these distribution results. Conclusion: We have here identified a class of ion channel modulators responsible for high ²²³Ra retention in the GI. Blockade of these channels results in altered pharmacokinetic of ²³³Ra - to reduce soft-tissue toxicity and increase on metastasis-therapy, resulting in an in situ increase in bone uptake. This combination therapy may provide a safe approach to reducing additional GI radiotoxicity while improving therapeutic effect to bone metastases. Acknowledgments: We would like to thank the DOE for ²²⁷Ac/Th supplies as well as NIH/NCI for supporting this research (R01-CA229893; R01-CA201035 DLJ Thorek) and ERF-SNMMI (Junior Faculty award 2015-Abou DS).

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