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Editorials
amine oxidase domain-containing histone demethylases (KDM1A, KDM1B), which are components of corepressor complexes that repress terminal differentiation genes. Nicotinamide adenine dinucleotide (NAD+) is a cofactor for Sirtuin histone deacetylases (SIRT1, SIRT2) that participate in the regulation of tissue stem-cell genes. In short, sensing for these mitochondrial outputs is another way in which exponential proliferation and onward differentiation are coupled (Figure 1B), powerfully demonstrated by the natu- ral experiment of neoplastic evolution: for example, recur- rent mutations in isocitrate dehydrogenase genes (IDH1, IDH2) in cancer reduce or antagonize AKG to decouple exponential proliferation and onward differentiation.10
Since CTP is disproportionately elevated in malignancy, and interventions that decrease CTP reconnect to onward differentiation, it can be theorized that CTP is a cofactor for corepressor proteins that repress terminal-differentiation genes. In fact, chromosome partitioning ParB proteins in bacteria, which also serve as platforms for DNA-condens- ing proteins (condensins), require CTP as a cofactor.11 The identity of the putative corepressor component in eukary- otic cells for which CTP is a cofactor is, however, unknown. An alternative theory is that CTP is a negative regulator of coactivators that activate terminal differentia- tion genes, whose identity is also unknown.
These observations are relevant to clinical translation. If a decrease in CTP and/or UTP mediates the induction of ter- minal differentiation by DHODH inhibitors, then the CTP/UTP content in cancer/leukemia cells is a candidate pharmacodynamic biomarker to guide the design of drug regimens. Moreover, the most likely mechanism for treat- ment failure can be anticipated: the pyrimidine metabolism network will respond automatically to preserve the amounts of CTP and/or UTP. For example, DHODH inhibitor-mediated depletion of CTP and UTP will relieve their allosteric inhibition of uridine cytidine kinase 2 (UCK2), which salvages cytidines and uridines from the extracellular environment, to automatically dampen any CTP/UTP decrease.12 CTP and/or UTP decreases can also be expected to trigger compensating shifts in the expression of key enzymes of pyrimidine metabolism: for example, ASLAN003 treatment of AML cells acutely upregulated expression of deoxycytidine kinase (DCK), which salvages deoxycytidines (Figure 1C). Treatment resistance that emerges automatically from the pyrimidine metabolism network in this way will be rapid and, importantly, will not be solved by simple escalation of DHODH-inhibitor dosages, since this would worsen the therapeutic index, which is a rationale for clinical development in the first place. Fortunately, compensatory metabolic responses can potentially be turned to advantage: DCK activates several oncotherapeutic prodrugs, e.g., decitabine that, like partial DHODH inhibition, can also operate in a non-cytotoxic, differentiation-based regime,13,14 and incorporation of DCK- dependent prodrugs can potentially be timed to DHODH inhibitor-mediated DCK upregulation. Other pyrimidine metabolism responses, however, could be adverse to such combinations: the pyrimidine metabolism enzyme SAM- and HD-containing deoxynucleoside triphosphate triphos- phohydrolase 1 (SAMHD1) was also automatically upregu- lated by ASLAN003 (Figure 1C) and catabolizes the active nucleotide forms of decitabine and cytarabine which are
routinely used to treat AML. Ultimately, therefore, candi- date solutions for resistance will require thorough experi- mentalevaluation.
Apoptosis/cytotoxicity is the standard pathway goal of oncotherapy, but is burdened by systemic toxicity and fre- quent futility, because of recurrent genetic attenuation of the p53-apoptosis axis in cancers and leukemias. DHODH inhibitors are appealing because of their potential to bolster a lagging inventory of p53-independent oncotherapeutics that cytoreduce malignancies by terminal differentiation instead. The pyrimidine metabolism network, however, will compensate automatically for reductions in CTP/UTP achieved by clinically viable doses of DHODH inhibitors. Our conditioned response to treatment failure is to escalate dosages toward more profound antimetabolite effects and cytotoxicity. Appreciation for, and fidelity to, terminal dif- ferentiation as the opportunity and pathway goal to be seized can help guard against these instincts – been there, done that! – and increase possibilities for clinical success.
Funding
National Heart, Lung and Blood Institute PO1 HL146372; National Cancer Institute P30 CA043703; National Cancer Institute RO1 CA204373
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