As there is a high risk of drug interactions with olaparib, it is important to investigate interactions with other drugs, including metformin, because it is administered to diabetic patients with ovarian cancer [20, 21]. Research has proven that metformin lowers the risk of liver, pancreatic, and breast cancers [21, 22], inhibits the growth of existing cancer cells, and reduces mortality in the course of ovarian, endometrial, and colorectal cancers [23]. Moreover, cell line studies have shown that metformin and olaparib synergistically inhibit tumor growth by blocking the cell cycle [13]. Research on animals assessing the interaction between sorafenib and metformin showed reduced exposure to the anticancer drug, but no changes in the PK parameters of metformin [24]. Moreover, as metformin is believed to inhibit the activation of genes encoding the CYP3A4 enzyme, a significant decrease in the metabolism of substrates (e.g., olaparib [7]) of this enzyme can be expected [25]. It is assumed that interactions occurring at the level of transporters increase the metformin concentration (and thus increase the risk of adverse effects) and decrease exposure to olaparib. This problem is important because it may be necessary to investigate the use of olaparib during the treatment of ovarian cancer and due to the presence of diabetes and the administration of metformin in cancer patients. There have been reports on the increased risk of ovarian cancer among diabetic patients. Diabetes, endometriosis, polycystic ovarian syndrome, as well as several genetic polymorphisms significantly increase the risk of ovarian cancer [26]. Cohort and nested case–control studies conducted by Lee showed that patients with diabetes were at statistically significantly higher risk of ovarian cancer (RR, 1.16; 95% CI, 1.01–1.33), without significant heterogeneity (I = 27; P = 0.172) [27].

For all these reasons, the pharmacodynamic mechanism of the interaction of metformin with olaparib should be investigated. Additionally, as olaparib raises the blood glucose level by blocking GLUT2 transporters, it results in overestimated fasting glucose level. Therefore, it is very likely that antihyperglycemic treatment will be implemented. It all shows that both drugs can be combined with each other, because type 2 diabetes, to which metformin is dedicated, poses a significant risk of ovarian cancer [7, 10, 11].

There is a high risk of drug interactions with olaparib. In vitro studies have shown that olaparib inhibits BCRP, OATP1B1, OCT1, OCT2, OAT3, MATE1, and MATE2K, which may result from increased exposure to the substrates of these transporters [1]. OCT1, MATE1, and MATE2K are important transporters for the pharmacokinetics of metformin. Metformin is a drug which does not have metabolites. It is excreted in an unchanged form with urine by glomerular filtration and tubular secretion. Metformin only minimally binds to blood proteins. However, it also binds to erythrocytes, which are its second distribution compartment. The role of transporters in the pharmacokinetics of metformin is very complex. OCT1 in enterocytes may influence the transport of metformin into the interstitial fluid. Additionally, the hepatic uptake is also mediated by OCT1. Therefore, the inhibition of OCT1 may decrease the effect of metformin. OCT2 is involved in the uptake of metformin from the blood into the kidney. MATE1 and MATE2K (efflux transporters) are responsible for the elimination of metformin from renal cells to the urine [28,29,30]. Additionally, the interaction with OCT2 in proximal tubule epithelial cells may increase the systemic disposition of metformin by reduced renal clearance. In our study, the co-administration of a single dose of olaparib with metformin significantly decreased the metformin clearance from 6.31 ± 1.48 l/h to 3.57 ± 1.26 l/h (p = 0.0014). The concomitant application of olaparib and metformin increased the metformin Cmax 2.8 times and its AUC 2.6 times. Such an increase in exposure to metformin may cause the risk of side effects, particularly in the gastrointestinal tract. According to some researchers, the weakening of the OCT1 function or the reduction of drug transport through OCT1 may result in gastrointestinal intolerance due to increased metformin concentration in the intestine [31, 32]. It is known that the gastrointestinal distress of metformin seems to be locally driven, hence it is hard to rationalize how do the higher systemic concentrations cause gastrointestinal side effects. Also, there is no clear evidence that bile excretion component of metformin increases in presence of olaparib. However, inhibition of the activity of the OCT1 transporter is an important issue.

Therefore, the effect of olaparib, which is an OCT1 and OCT2 inhibitor, on the increased risk of GI intolerance of patients taking metformin cannot excluded.

It is noteworthy that in the absence of hypersensitivity to metformin, it is used as a first-choice drug in the treatment of type 2 diabetes. Additionally, metformin has been proved to reduce the risk of liver, pancreatic, and breast cancers [21, 22], inhibit the growth of existing cancer cells, and reduce mortality in the course of ovarian, endometrial, and colorectal cancers [23]. Moreover, cell line studies have shown that metformin and olaparib synergistically inhibit tumor growth by blocking the cell cycle [13]. Therefore, metformin is a promising drug in the treatment of cancer.

Metformin is not expected to be involved in many drug–drug interactions (DDIs) but there are studies showing that it has the potential to be the perpetrator in DDIs. Metformin reduced the Cmax and AUC24 of aliskiren but the changes were not significant, so clinical DDIs are not expected. There have been studies showing that metformin affects phenprocoumon and warfarin. It is known that the Cmax and AUC24 of trospium decreased when it was combined with metformin – probably metformin can inhibit the oral absorption of the drug. On the other hand, metformin was found to increase the exposure to topiramate [33]. Vuu et al. [34] observed that the co-administration of metformin and sotorasib did not affect the sotorasib exposure to a clinically significant extent. It also did not affect the hypoglycemic effect of metformin, although it was different from the one observed in vitro and its duration was shorter.

Chinese researchers hypothesized that the combination of sorafenib and metformin may have a synergistic effect in the treatment of colorectal cancer while reducing the severity of side effects [35]. However, when vandetanib is combined with metformin, the latter may require additional monitoring and periodic dose escalation [36]. The authors of the METAL (METformin in Advanced Lung Cancer) study [37], which was a phase I-II trial, hypothesized that the administration of metformin to non-diabetic patients may revert resistance to gefitinib, which is a selective epidermal growth factor receptor (EGFR) and tyrosine kinase inhibitor applied in non-small cell lung cancer. The researchers observed that a stable blood glucose level was maintained in the non-diabetic population. At the same time, during the 30-week observation period the neoplastic disease became stabilized in 50% of the patients. The combination of metformin with gefitinib inhibits cell proliferation and induces apoptosis, particularly in cell lines harboring the wild-type LKB1 gene. This dependence can also be observed in another tyrosine kinase inhibitor – erlotinib. The time-to-progression median was 20 weeks. This effect may have been caused by the fact that metformin may activate AMP-activated protein kinase and thus inhibit the mTOR and block the MAPK signaling. The relationships between metformin and tyrosine kinase inhibitor are constantly being investigated [37].

Clinical trials on metformin have not shown any influence of this drug on the efficacy of the following medications: alogliptin, dapagliflozin, dutogliptin, gemigliptin, linagliptin, lobeglitazone, rosiglitazone, rosuvastatin, saxagliptin, sitagliptin, and vildagliptin [33]. As metformin is believed to inhibit the activation of the genes encoding the CYP3A4 enzyme, a significant decrease in the metabolism of substrates (e.g., olaparib [7]) of this enzyme can be expected [25]. Gralewska et al. found that the treatment with olaparib and metformin increased oxidative stress and decreased the mitochondrial membrane potential. The co-administration of metformin and olaparib may result in almost two times greater early apoptosis than when the drugs are administered individually. After the co-administration of olaparib with metformin the percentage of late apoptotic cells was significantly higher than when the drugs were given separately (28.4% for co-administration vs. 5.1% for olaparib and 8.2% for metformin) [6]. Another study showed that biguanides in combination with PARP inhibitors synergistically reduced the epithelial-mesenchymal transition, proliferation, and survival of ovarian drug-resistant cancer cells [38].

Our research showed that a single dose of metformin did not have inhibitory effect on olaparib and did not affect its PK parameters. However, olaparib significantly changed the pharmacokinetics of metformin. The Cmax of metformin increased by 177.8%, whereas the Vd/F and Cl/F of metformin decreased. There were no significant differences between the two groups (metformin co-administered with olaparib and metformin administered alone) in the other pharmacokinetic parameters, including tmax (p = 0.0580), ka (p = 0.1170), and t0.5 (p = 0.0587. The values of the IMET+OLA/IIMET ratio for Cmax, AUC0-t, and AUC0→∞ were 2.78, 2.59, and 1.74, respectively. Investigations in human in vitro systems indicated phase I metabolism of olaparib was CYP mediated and that CYP3A4 and 3A5 were the dominant metabolic enzymes. As expression of CYPs 3A4 and 3A5 is highly variable in human and olaparib clearance in human was primarily metabolic, this may explain some of the variability observed in clinical pharmacokinetics. Perhaps, in the study, the high variability contributed to the lack of statistically significant differences in the PK parameters of olaparib.

There were some limitations to our study, such as the small size of the sample, which was limited by the Local Ethics Committee (No. 45/2022, of 27 May 2022). The lack of using a model is also a significant limitation of the study. Another limitation was the fact that both drugs (not only metformin but also olaparib) were administered only once and at the same dose. If the experiment had been continued to the steady state (as in patients), there might have been changes in the olaparib PK as well. If the experiment had been conducted on pre-diabetic or diabetic animals, the effect of the pathological condition on the PK of both drugs might also have been observed.