5. Overcoming cDDP Resistance by Copper Chelators
Many human cancers have increased Cu contents that are implicated in tumor angiogenesis, proliferation, and migration. Likewise, Sp1 expression is also frequently elevated in many human cancers
[58]. Many preclinical studies have demonstrated that Cu chelators suppress tumor growth in cultured cell models
[59] and in animal tumor models (see review
[60] and references therein). Cu chelators used in these studies include tetrathiomolybdate (TM)
[60], D-penicillamine (D-Pen)
[60], trientine triethylenetetraminee dihydrochloride
[60], disulfram (DSF)
[61][62], and elesclomol (STA-4783)
[63] (see reference
[60] for the chemical structures and their clinical trials for some of these Cu chelators). TM, D-pen, and trientine are traditional medicines used in treating Wilson’s disease, whereas DSF is a conventional anti-alcoholism drug, and STA-4783 is an anti-neurodegenerative agent recently found to have activity of escorting Cu to the brain in mouse Menke’s disease model
[64].
The Cu-Sp1-hCtr1 cycle underscores that the cellular levels of Cu(I), Sp1, and hCtr1 are mutually regulated. The capacities of cellular hCtr1 levels that can be regulated by Cu(I) depletion are constrained and may vary among cell types. For example, cells with reduced basal hCtr1 levels have higher magnitudes of hCtr1 upregulation by Cu chelators than those with elevated basal hCtr1 levels. Because most cDDP-resistant variants are associated with reduced hCtr1 expression
[65], these results suggest higher magnitude of hCtr1 induction by chelators in cDDP-resistant cells than in their drug-sensitive counterparts. This predication was confirmed in our study using three cultured cell models and three different Cu-lowering agents (trientine, D-pen, and TM); cDDP-resistant cancer cells exhibit a greater magnitude of hCtr1 upregulation by the Cu-lowering agents as compared with their drug-sensitive counterparts. These observations indicate that reversal of cDDP resistance is independent of Cu-lowering agents and of cell line-specificities
[66]. Another study using high-throughput screening identified that disulfiram exhibits synergistic effects with cDDP in bladder cancer cells
[67]. Moreover, other naturally occurring Cu-lowering products include carnosine dipeptide (alanyl-L-histidine), which has been reported to modulate the Sp1-hCtr1-Cu homeostasis system
[68], and curcumin (a product of plant
Curcuma longa used in food flavoring), which enhances the binding of Sp1 to
Ctr1 and Sp1 promoters, and thus induces Ctr1 expression and chemosensitization to cDDP treatment
[69]
Based on these preclinical observations, two clinical trials have been carried out in testing the efficacies of Cu-lowering agents as enhancers in Pt drug cancer chemotherapy: one (at MD Anderson Cancer Center) involved carboplatin plus trientine in 55 patients with advanced malignancies, 45 of which had prior failure in Pt drug treatment. The results showed that about 19% of patients (
n = 9) who maintained low serum Cu levels after the treatments had significantly longer median PFS (
p = 0.001) and OS (
p = 0.03), as compared with those patients (
n = 38) who did not
[70][71]. The other study (at the National Cheng Kung University, Taiwan) involved carboplatin plus trientine and pegylated lyposomal doxorubicin in 18 Asian epithelial ovarian tubal and peritoneal cancers. The clinical benefit rate was 33.3 and 50.0% in the Pt-resistant and the partially Pt-sensitive group, respectively
[72].
Savage therapy of Pt-resistant patients is well-known to produce low response rates (less than 10%)
[73][74]. These studies provided first-in-human encouraging results that warrant further investigations.