Here, we performed detailed analyses of the PK profile of MitoTam and subanalyses of the clinically relevant endpoints of MitoTam phase I/Ib trial from the perspective of PK findings. To expand on the main PK results, we performed additional analyses to reflect the two treatment schemas and clinico-pathological variables, including the toxicity of MitoTam. Our approach was based on comparative statistical analysis of the calculated PK parameters, the role of the treatment regimen, the number of treatment cycles, the dose of MitoTam, sex, and baseline diagnosis. We paid special attention to whether the exceptional treatment outcomes in regimen 2 (CBR 78%) and in a virtual subcohort of patients with RCC (CBR 83%) were statistically significant or a random finding. All of these variables were tested to better understand their impact on the PK of MitoTam.
The PK analysis showed a low extraction ratio and rapid distribution to the periphery. Most of the PK profiles indicated possible redistribution of MitoTam from the tissues or protein binding back into the serum, because secondary peaks in serum concentrations were occasionally observed. These secondary peaks were first observed in the 1.5 mg/kg cohort in phase I and subsequently observed in all cohorts in phase Ib. Preclinical biodistribution studies in animals [8] showed that MitoTam mostly accumulated in the kidneys, myocardium, lungs, and liver. Increased metabolism and high concentrations of the N-desmethyl MitoTam metabolite were observed in the liver and duodenum within 24 h post-dose, whereas the concentration in the kidneys increased steadily over a 1-week period, suggestive of accumulation rather than metabolization in this organ. The preclinical findings might help explain which tissues are the likely source of the secondary MitoTam peak. Overall, our clinical observations support the preclinical findings that MitoTam is excreted through the liver and bile ducts rather than via the kidneys.
MitoTam was detectable as long as 168 h after the start of the infusion, supporting the idea of a large Vz and high tissue affinity. We believe that the pig model used in preclinical studies [8] adequately addresses and explains the large volume of MitoTam distribution, however, clinically it is not feasible to confirm high tissue affinity since it is ethically unacceptable to take samples and evaluate PK parameters from patient tumors, liver and/or kidneys.
Regarding the study’s primary objective—to determine the optimal and safe dose for further testing—we evaluated the relationship between the MitoTam dose and PK parameters. Elevated AUC0 − t, Cmax, T½α and CL were recorded in the 4.0, 5.0, and 6.0 mg/kg cohorts in phase I and in regimen 3 of phase Ib. However, the differences in PK parameters between regimens 2 and 3 in phase Ib were generally not significant (AUC0 − t, Cmax, and CL) with the exception of T½α. The prolonged serum half-life T½α was not related to the serum total protein and albumin concentrations. Our hypothesis that the elevated T½α at doses above 4.0 mg/kg may be related to depleted albumin binding and a subsequent greater free fraction of MitoTam proved to be wrong. Rather, it seems that the significantly longer T½α at doses above 4.0 mg/kg is related to the already exhausted terminal elimination process, which correlates with our clinical observations. We can conclude that the dose of 3.0 mg/kg (in regimen 2) is optimal for further testing from clinical and pharmacological perspectives.
The AEs at the dose of 3.0 mg/kg were predominantly G1/2 anemia [9], a promising finding when compared to the safety profiles of other mitochondrial agents [2–6]. The risk of TE, which occurred in 13% of patients in phase Ib, may be related to the greater biodistribution of MitoTam in lung tissue, as observed in the preclinical model, and the lipophilic properties of the drug. Nevertheless, risk factors such as prior history of TE disease, the malignancy itself, and a long presence of an inadequate venous route (i.e. peripherally inserted central catheter) should be considered.
Regarding the efficacy of MitoTam, preclinical studies demonstrated high anticancer activity in several mouse models of cancer [12, 13]. Our hypothesis that the CBR would be related to the PK parameters, primarily the AUC0 − t and Cmax, in regimens 2 and 3, was not confirmed. We think this is due to the high Vz of MitoTam, the high permeability of MitoTam into cells, and its high binding to tissue components. We thus conclude that the high CBR of MitoTam in patients with RCC is due to preferential accumulation in the kidneys. The number of patients whose tumors originated in the mesodermal layer is too small to conclude whether MitoTam is effective in mesodermal-layer-derived tumors other than RCC.
We searched for articles published in PubMed up to March 2023, and we found 88 studies in which cancers were targeted with mitochondrial inhibitors. Only 12 were active studies, almost exclusively in phases I or II. From this perspective, the successful antitumor effects of MitoTam observed in this trial hold great promise.
The limitations of our study were discussed previously [9]. Some limitations, particularly the small number of participants, are inherent to phase I clinical trials. We are aware that the exposure-efficacy relationship is not well supported by data due to the small number of patients, resulting in confidence intervals for these comparisons being too wide and overlapping. The uneven representation of diagnoses between the study cohorts was due to the random recruitment of patients in the phase I trial, which was not powered to assess efficacy outcomes. The main technical limitation of our study was the use of two alternative treatment schemes in three regimens, which makes it difficult to compare the results among the patient groups. Therefore, we focused our PK analysis on the cohorts/regimens using a weekly dosing schedule. We believe that the results provide strong statistical evidence of clinical activity of MitoTam in tumors originating in the ME, which should be considered in future prospective phase II studies. Further confirmation of the clinical activity of MitoTam in patients with RCC is warranted.