Using this approach, we have identified a number of pure inhibitors based on their high potency in inhibiting CYP24 activity and their low VDR transcriptional activity. The identified inhibitors (e.g., CTA091 IC50<10 nM; Figure 3) compared favorably to ketoconazole, which itself has a respectable IC50 in the 400–600 nM range. These inhibitors were selected because they exhibited low or no residual ability to induce CYP24 gene expression. In addition, inhibitors like CTA091 do not inhibit the closely related cytochrome P450s such as CYP27A (which metabolizes vitamin D3) or CYP27B1 (which metabolizes calcifediol).
Epidemiological studies have provided strong evidence that adequate endogenous supply of calcitriol is associated with reduced incidence of certain cancers including carcinomas of the prostate, colon, and breast.
Studies conducted both in vitro and in vivo support calcitriol’s role in regulating proliferation, differentiation, apoptosis, angiogenesis, and the metastatic processes involved in tumorigenesis.
CYP24 in Oncology
Rapid catabolism of calcitriol by CYP24 is a significant barrier to the hormone’s potential therapeutic use in cancer treatment. Calcitriol upregulates CYP24 gene expression, thereby promoting its own catabolism and leading to gradual development of treatment resistance.
Interestingly, recent analyses of several human cancer tissues (colorectal, gastrointestinal, prostate, and skin) at the molecular level have demonstrated abnormally elevated CYP24 expression compared to normal tissue and a correlation between high CYP24 expression and tumorigenicity. These observations suggest that specific CYP24 inhibition may have therapeutic potential in cancer prevention and treatment. In addition, CYP24 inhibition may also enhance the efficacy and pharmacokinetic profile of calcitriol and its analogs by limiting their catabolic breakdown.
CYP24 in CKD
In CKD, secondary hyperparathyroidism frequently arises as blood levels of calcitriol decline. Blood parathyroid hormone (PTH) levels, normally controlled by calcitriol, begin to increase as the capacity of the failing kidneys to produce adequate calcitriol steadily diminishes.
Exogenous calcitriol, doxercalciferol, and paricalcitol effectively lower PTH but also induce CYP24, especially when administered by bolus intravenous injection. It is likely that elevated CYP24 plays a central role in contributing to the widespread vitamin D insufficiency observed in CKD patients, a problem that has recently been linked to increased patient mortality.
CYP24 Inhibitors in the Clinic
Pure CYP24 inhibitors can provide clinicians in several disciplines with new tools to use directly or in conjunction with vitamin D replacement therapies to treat vitamin D responsive diseases. Blocking CYP24 activity in hyperproliferative diseases like cancer and psoriasis can enable optimal penetration of target tissue with vitamin D hormones. Limiting CYP24 activity in a progressive disease like CKD can attenuate the rapid decline in vitamin D status, which is a key factor in the overall health of these patients. Vitamin D therapy for other diseases may also benefit from coadministration of CYP24 inhibitors.
Finally, the use of dual-action vitamin D compounds that simultaneously act as potent agonists of VDR transcriptional activity and as potent inhibitors of CYP24 activity, may serve as self-potentiating therapies that prove superior to the single action compounds currently serving as therapies of first choice.