The phase 1 pilot study performed at MD Anderson Cancer Center to evaluate the concept of using Cu chelators for improving the effectiveness of Pt drugs involved five patients with Pt-resistant high-grade epithelial ovarian cancers. These patients were treated with trientiene and CBDCA. Two patients experienced severe adverse events that included myelosuppression, especially anemia requiring transfusion. Dose-limiting toxicity was not observed within the first 28 days. After two cycles of therapy, partial remission was achieved in one patient (10+ months), stable disease in three patients (2, 3.5+ and 5 months, respectively), and progressive disease in one patient. Better tumor responses were associated with greater decreases in Cu levels using the surrogate biomarkers serum ceruloplasmin (16
). This study provides the first-in-human preliminary data showing that Pt-resistance in tumors could be at least partially overcome in some patients through the use of a Cu-lowering agent. Further study using larger patient population with improved strategies (see below) is warranted. In the meantime, several relevant issues are discussed below:
First, Cu-lowering agents such as trientine and D
-penicillamine have been used for more than four decades for treating Cu toxicosis in Wilson’s disease, a genetic disorder caused by defects in ATP7B
. Another Cu-lowering agent, tetrathiomolybdate (TM), has been in clinical trials as an antitumor agent. Cu is a cofactor required for several angiogenic mediators, including VEGF, bFGF, IL-1 and IL-8 (24
). It has been shown that many patients with malignancies of the breast, colon, lung, prostate, and brain, display elevated Cu contents in their serum and tumors (24
). Cu-lowering agents have been used in monotherapy by targeting the angiogenic growth mechanisms of the tumors (25
). Combining Cu-lowering agents and Pt drugs in cancer chemotherapy may have additive antitumor effects (and may also have additive cytotoxicities, see below). This strategy is essentially a new use of old drugs, and therefore, if proven, would be a low cost treatment option for cancer patients.
Second, although the trientine/CBDCA protocol is targeting Pt-resistant tumors with reduced hCtr1 expression, however, mechanisms of cDDP resistance are multifactoral and reduced hCtr1 alone is not the only reason for drug resistance. Other factors including elevated ATP7A/ATP7B transporters (26
) and hCtr2 (27
), may also affect the treatment efficacy. To maximize the effect of Cu-chelators, determination of biomarkers associated with various cDDP resistance mechanisms should be very helpful to stratify patients who may benefit most from the treatment.
Third, the effectiveness of Cu-lowering agents in enhancing hCtr1 expression in vivo
is a critical issue. A previous study of rats fed a Cu-deficient diet failed to show increased Ctr1 mRNA levels in livers and small intestines despite a substantial loss in Cu levels in these organs (69 ~ 89% reduction), as correspondingly compared with those in animals fed Cu-adequate diet (28
). A cervical tumor model developed in transgenic HPV16/E2 mouse showed elevated mCtr1 protein levels as compared with those in the normal cervix. TM treatments did not show further induction of mCtr1 expression in the tumor lesions of transgenic mice, nor in the cervix of the wild-type animals (10
). In contrast, increased mCtr1 expression was found in several organs (kidney, duodenum, brain) in Cu-deficient, postnatal day-16, mice fed Cu-deficient diets (29
). Aside from the technical issues related to Ctr1 measurement as mentioned and different tissue sources were used in these studies, this inconsistency (#4) may reflect the complexity of in vivo
Ctr1 regulation mechanisms. Further investigations are needed to address the following important issues: (i) The in vivo
Ctr1 regulation mechanism by Cu deprivation may be more stringent than the in vitro
system using cultured cells. (ii) Elevated Ctr1 expression may be an intrinsic mechanism associated with tumor development. This may explain why elevated Cu levels were observed in many human malignancies (24
) and in the HPV16/E2 cervical tumors (10
). Moreover, failure to further induce tumoric mCtr1 expression in the HPV16/E2 tumors by TM may be explained that these tumors already express elevated levels of mCtr1 (). (iii) The effectiveness of Cu-lowering agents in enhancing cDDP sensitivity may be tissue-specific: tissues expressing high levels of hCtr1 may be less sensitive to Cu chelation-induced Ctr1 expression than those expressing reduced hCtr1 levels. A better understanding of hCtr1 expression in response to Cu chelation treatment in different settings may eventually lead to the use of hCtr1 inducibility as a predictor for the treatment outcome of Pt drug therapy.
Fourth, drug-induced toxicity is also an important issue. Both Cu chelators and cDDP are redox-active compounds and cause oxidative damage to cells. Combination therapy may exacerbate toxicity resulting in a myriad of pathophysiological consequences. In cancer chemotherapy, although cDDP is known to cause toxicities in many organs including kidney, nerve, ear, and bone marrow, CBDCA which is known as the second generation antitumor Pt drug, displays much reduced toxicities in these organs except bone marrow. Cu chelators are also known to cause bone marrow damage associated with anemia/leukopenia and worsen neurologic symptoms in Wilson’s disease (25
). Accordingly, bone morrow seems to be the most likely target for the adverse events associated with Cu chelator/CBDCA combination therapy. This indeed occurred in our preliminary trientine/CBDCA trial which involved only five advanced ovarian cancer patients (16
Clinical experience shows that the adverse effects inflicted by CBDCA and by Cu chelator therapy could be overcome when treatments were discontinued, implicating that the treatment-induced adverse events may be manageable. In this regard, some suggestions may be offered: (i) by modifying the treatment schedule such as sequential administration of a Cu chelator first to induce hCtr1 expression followed by CBDCA treatment to optimize the antitumor efficacy; (ii) by carefully designing a treatment holiday to minimize the cytotoxicities; and (iii) by critically evaluating the combinations between CBDCA and various Cu-lowering agents. These strategies may eventually improve the overall therapeutic index for the use of Cu chelator and CBDCA combinations.
And fifth, recent studies have demonstrated that cancer-initiating cells in human malignancies are resistant to cDDP (30
). Although these cDDP-resistant cells may be in small population and present in special niches, they are highly tumorgenic in the immunocompromised mice. Currently we know very little about cDDP resistance mechanisms associated with this tumor population. This drug-resistant population may be the most difficult target for complete elimination by chemotherapy. The role of hCtr1 in cDDP resistance in these cancer-initiating cells remains to be investigated.