Statins Potentiate Aminopeptidase Inhibitor (pro)Drug Activity in Acute Myeloid Leukemia Cells

Tosedostat represents a next generation oral aminopeptidase inhibitor prodrug that has recently demonstrated promising activity in elderly patients with relapsed and refractory acute myeloid leukemia (AML). This prodrug is bio-activated to its hydrophilic active metabolite by intracellular esterases including carboxylesterase 1 (CES1) hence enhancing its cellular retention and promoting targeting of multiple aminopeptidases.

Aminopeptidase inhibition provokes an amino acid deprivation response, inhibition of mTOR activity and blockade of protein synthesis. The fact that CES1 also has an important physiological function in cholesterol metabolism (i.e. conversion of cholesteryl-esters to cholesterol), and the notion that AML cells display an aberrant cholesterol metabolism, prompted us to explore whether or not statins, as inhibitors of the mevalonate-cholesterol biosynthesis pathway, can potentiate the cytotoxic activity of aminopeptidase inhibitor prodrugs.

Here, we discovered that the antitumor activity of CHR2863, a close structural analogue of Tosedostat, is markedly potentiated by non-toxic and clinically achievable concentrations of statins. These findings were supported by the following lines of experimental evidence; (i) A strong synergy in cell growth inhibition (combination indices < 0.1) of CHR2863 with various statins (simvastatin, fluvastatin, lovastatin and pravastatin) was demonstrated for U937 AML cells and U937 sublines displaying acquired resistance to CHR2863. (ii) This potent synergy between CHR2863 and simvastatin was also observed with a spectrum of human AML cell lines including THP1, MV4-11, and KG1, but not with acute lymphocytic leukemia (CCRF-CEM) or solid tumor cell lines (MCF7 breast cancer, KB nasopharyngeal carcinoma and SW1573 non-small cell lung cancer). (iii) The synergistic effects with non-toxic concentrations of statins were specific for aminopeptidase inhibitors, either prodrugs (CHR2863) or direct inhibitors (e.g. Bestatin), but not for other unrelated cytotoxic agents, including daunorubicin and an esterase-dependent prodrug of a histone deacetylase inhibitor. (iv) The synergy of CHR2863 with statins was also corroborated by enhanced apoptosis induction and cell cycle arrest which increased the sub-G1 fraction. (v) Statin potentiation of CHR2863 activity was abrogated by co-administration with mevalonate or farnesylpyrophosphate, and partly by geranylgeranylpyrophosphate, suggesting the involvement of protein prenylation. Consistenly, the farnesyltransferase inhibitor FTI-277 also synergized with CHR2863 activity. (vi) Mechanistically, non-toxic concentrations of simvastatin impaired Rheb prenylation which is required for lysosomal membrane association and activation of mTOR. Hence, we propose that the dual inhibitory effect of impaired Rheb prenylation by statins and CHR2863-induced mTOR inhibition achieves a potent synergistic inhibition on human AML cells. (viii) Finally, retrospective analysis in a clinical trial of relapsed and refractory elderly AML patients treated with combination chemotherapy including Tosedostat (Cortes et al, Lancet Oncol 2013), revealed a clinical benefit for patients who used a statin. AML patients on statins (n=8) and Tosedostat (switching from 240 to 120 mg/m²) had a 50% probability of 6 months survival as compared to 3 months survival for patients not on statins (n= 27). The two-year overall survival (20%) was not impacted by statin administration.

Collectively, these novel findings uncover a potent therapeutic combination of statins and aminopeptidase inhibitors for the treatment of AML. This drug combination warrants further clinical evaluation, particularly in the context of elderly patients using statins for other comorbidities.