Introduction

Epithelial ovarian cancer has the highest mortality of all gynecological cancers [1]. The disease is often diagnosed at a late stage because symptoms only develop once the disease has spread throughout the abdominal cavity. First line therapy for advanced disease consists of complete debulking surgery in combination with platinum-based chemotherapy [2]. Initial complete response to therapy can be achieved. However, the majority of patients ultimately develop recurrent disease, with over 50% of women diagnosed with epithelial ovarian cancer eventually dying of their disease [3]. Novel targets are needed in ovarian cancer and targeting metabolic reprogramming is of interest [4, 5]. Mutations in the key tumor suppressor gene and metabolic regulator p53 are the most frequent genetic abnormality occurring in ovarian cancer [6,7,8]. Furthermore, the mammalian target of rapamycin (mTOR) pathway is overactivated in approximately 70% of ovarian cancers and regulates protein translation of cell growth regulators such as cyclin D1, hypoxia inducible factor 1α (HIF1α) and MYC, all essential for survival under cellular stress [9,10,11].

Metformin is a biguanide widely used in patients with type 2 diabetes mellitus (T2DM). It improves glycemic control by decreasing insulin resistance, reducing hepatic gluconeogenesis and inhibiting gastro-intestinal glucose resorption [12]. In cancer cells, metformin inhibits mTOR through activation of AMP-activated protein kinase (AMPK) resulting in reduced cellular proliferation [13]. In addition, metformin reduces cellular respiration by inhibiting complex 1 of the mitochondrial respiratory chain limiting the cancer cell’s metabolic plasticity [14,15,16]. Metformin-treated cancer cells compensate for suppression of oxidative phosphorylation by enhancing glycolysis. This metabolic conversion appears to be p53-dependent [17]. Therefore, in the absence of functional p53, as is usually the case in high grade serous ovarian cancer (HGSOC), cancer cells might be unable to compensate for metformin-induced suppression of oxidative metabolism [36,37,38,39]. The dose-escalation scheme used in our study, likely contributed to the relatively high RP2D (3000 mg/day). The characteristics of previous trials can be found in supplementary Table 5 [36,37,38,39,40,41,42,43,44,45,46,47,48].

The combination of metformin with carboplatin and paclitaxel was tolerable and no dose-limiting toxicities occurred. Diarrhea is a well-known side effect of metformin, occurring in 20% of metformin treated diabetic patients leading to discontinuation in 5% of patients [49, 50]. Diarrhea occurred in eight patients in our study (53%), all grade 1–2. Although this is comparable to rates observed in other metformin combination studies it must be considered that diarrhea, even at low grades, can have a serious impact on health-related quality of life, and reduce treatment adherence. Hypomagnesemia is reported in 18% of type 2 diabetes mellitus patients receiving metformin [51], and even more frequently in patients receiving carboplatin (up to 46%) [52]. We therefore expected to observe an increased frequency of hypomagnesemia. Hypomagnesemia indeed occurred in 12 of 15 patients (80%) in our trial. However, in the majority of patients this was mild (11 patients grade 1–2) and all patients were treated with oral supplementation only. Other frequently occurring adverse events were related to myelosuppression, with frequencies not higher than expected for carboplatin and paclitaxel chemotherapy.

Metformin increased the Pt AUC by 22% and reduced Pt clearance by28%. Due to the small number of patients per dose group, it was not possible to determine whether metformin had a dose-dependent effect on Pt AUC and clearance. There were no other obvious factors explaining the difference in Pt AUC and Pt clearance between cycle 1 and cycle 2. GFR did not change significantly, changes in albumin levels were not considered to be of influence because carboplatin does not bind to protein in vitro [53] and corrections for body weight changes were incorporated in the calculation of the carboplatin doses in cycle 1 and cycle 2.

The metformin-induced reduction in renal clearance of Pt could be due to a direct effect of metformin on the tubular resorption of Pt, via its copper-binding properties [54, 55]. Copper transporters 1 and 2 (CTR1 and CTR2) regulate both copper-transport and transport of carboplatin as well as cisplatin into the cell [56, 57]. Copper-deficiency can be induced by metformin binding to copper which may result in upregulation of CTR1 as a compensatory mechanism [58]. Upregulation of CTR1 in kidney tubular cells may result in improved retention of carboplatin resulting in decreased clearance. Intracellular copper deficiency in cancer cells may also enhance carboplatin uptake due to upregulation of copper transporters [59]. Five patients with platinum resistant HGSOC were treated with carboplatin in combination with the copper chelator trientine yielding a partial response in one patient and stable disease in three patients [60]. Metformin has indeed been shown to reverse platinum-resistance of ovarian cancer cells through various mechanisms, but copper transporters have not been evaluated in these models [61, 62]. The majority of preclinical studies on anti-cancer effects of metformin used higher concentrations of metformin than those that can be achieved in patients with diabetes [63]. However, metformin has been shown to accumulate in tumor tissue, potentially due to the acidic tumor microenvironment, which could well result in intratumoral concentrations in the ranges studied preclinically [64, 65].

In conclusion, our study showed that the addition of metformin to carboplatin/paclitaxel in advanced ovarian cancer is feasible and well tolerated, with a RP2D of 1000 mg tds. A potential PK interaction of metformin with carboplatin was identified.