Cabozantinib and Vandetanib in medullary thyroid carcinoma: mitochondrial function and its potential as a therapeutic target towards novel strategies to design anti-CSCs drugs

Dear Editors

We read with great interest the recent publication of Starenki et al¹ examining medullary thyroid carcinoma (MTC) resistance by having generated drug-resistant subpopulations of human MTC cell lines via prolonged exposure to vandetanib and cabozantinib, as they bring up excellent discussion topics on the subject. In this respect, we would like to highlight some issues, as we are now well and truly in the era of molecular thyroid cancer medicine.

Trying to understand the pathophysiology of thyroid cancer prompted clinical trials to assess the antitumor activity of TKIs that inhibit the RET kinase, VEGFR, and other kinases. The mechanisms of primary resistance and of resistance occurring during treatment are indisputably poorly understood. Progression might be associated with insufficient drug doses requiring pharmacokinetic studies or with intra-tumor mechanisms. A RET mutation at codon 804 can potentially induce resistance to vandetanib² Further studies of tumor samples and in experimental models need to correlate drug efficacy with the genetic defects present in the tumor. Additionally, tests need to be done regarding other drug combinations in patients with medullary thyroid cancer along with immunotherapy used either alone or in combination with kinase inhibitors.²

Vandetanib was granted EU approval on the 17th of February 2012 for the treatment of aggressive and symptomatic MTC in adult patients with unresectable locally advanced or metastatic disease, and is also indicated for the treatment of children between 5 to 18 years of age. Interestingly, vandetanib has been included consecutively by Prescrire International³ in their list of drugs to avoid in oncology for 2014, 2015 and 2016, due to short follow-up of only 2 years, and too many patients lost to follow-up providing inadequate data for the estimation of progression-free survival. Assessment reports from the European Medicines Agency on vandetanib have shown that one third of the patients present serious adverse effects including diarrhea and other gastrointestinal disorders, cardiovascular disorders, along with cutaneous, neuropsychological, haemorrhagic and metabolic disorders, visual disturbances, pneumonia, hypertension, increased risk of interstitial lung disease, prolonged QT interval and torsades de pointes, renal failure, and sudden death.⁴

Cabozantinib like vandetanib inhibits the activity of various tyrosine kinases, which are implicated in angiogenesis processes and tumor growth. Assessment reports from the European Medicines Agency⁵ have reported various adverse effects with 69% of patients in the cabozantinib groups versus 33% in the placebo groups having grade 3 or 4 events including haemorrhage, gastrointestinal perforation and fistulae, diarrhea and other gastrointestinal disorders, hypertension, venous and arterial thrombosis, skin rash, oral pain and stomatitis, fatigue, hand-foot syndrome (50% of patients in the cabozantinib groups versus 2% in the placebo groups), and more rarely, osteonecrosis of the jaw, reversible posterior leukoencephalopathy, and decreased wound healing.⁵ Frequencies regarding mortality and deaths unrelated to tumor progression have been increased including causes like sudden death, bleeding, gastrointestinal fistulae, sepsis, cardiopulmonary failure, and respiratory failure. Regarding the clinical efficacy of the drug, a placebo-controlled trial having enrolled 330 patients with advanced-stage or metastatic medullary cancer⁶ showed that, despite improved radiological and laboratory parameters, neither survival (29% in 8 months versus 49% in 12 months) nor symptoms were improved. Patients’ quality of life worsened due to troublesome diarrhea. In fact, 95% of the patients presented with metastasized disease in a timeframe of 4 years after their randomization in the trial, although 90% were subjected to thyroidectomy, 50% to radiation therapy and 40% to chemotherapy, whereas 10% have additionally received vandetanib and 10% another tyrosine kinase inhibitor. More withdrawals have been observed due to serious adverse events occurrence, requiring dose reductions with only a 14% of the patients remaining on their initial dose until study termination. Furthermore, both the FDA and EMA requested additional studies on efficacy and toxic effects of cabozantinib when administered at a lower initial dosage (60 mg/day vs 140 mg/day). Multiple pharmacodynamics interactions have been observed confirming the drugs teratogenic action, and impaired male and female fertility, overall presenting an unfavourable harm-benefit balance in medullary thyroid cancer.^6,7 Therefore, a question arises on why use a medication that can affect the patient's quality of life due to the serious adverse events if it does not improve the overall survival?

The authors¹ address a potentially important aspect of MTC resistance possibly due to mechanisms affecting RET downstream leading to cross-resistance, supporting that combination of vandetanib and cabozantinib with a mitochondria-targeted agent may have potential anti-MTC efficacy. As advanced-stage or metastatic tumors do not respond to radiation therapy and novel chemotherapy, a single drug against cancer may not be effective as (CCs) cancer cells develop multiple drug resistance along the treatment, whereas they are subject to reactivation. Conventional treatments induce apoptosis or autophagy in the CCs but cannot eradicate the (CSCs) cancer stem cells, which can resume growth more aggressively. Tumor initiation and progression is attributed to this group of cells exhibiting a stem phenotype that maintains tumors through continuous generation of progenitor cells,⁸ enhancing tumor initiation, hypoxic tolerance, capacity for neovessel induction, chemoresistance, and metastasis.⁹ Mitochondria-targeted agents are more effective compared to other agents in triggering CSCs and general CCs apoptosis, via mitochondrial dysfunction due to alterations in the mitochondrial metabolism in cancer cells attributable to their dependence on glycolytic intermediates. Therefore, inhibiting cancer-specific modifications in mitochondrial metabolism, increasing reactive oxygen species production, or stimulating mitochondrial permeabilization transition could become new therapeutic strategies to activate cell death in CSCs and general CCs.^10,11

This is better understood when one considers that several protective CSC pathways include the mitochondrial ABC transporters which are implicated in redox homeostasis, oxidative phosphorylation (OXPHOS) triggering mitochondrial ROS production from electron transport chain complexes¹² and subsequently promoting transition to oxidative metabolism. Their deficiency decreases mitochondrial potential and provokes DNA damage in the mitochondria, whereas the redox dysregulation can impact CSCs survival after chemotherapy, subsequently showing that ABC transporter activity decrease may lead to combatting drug resistance. Processes like the Epithelial-Mesenchymal Transition (EMT) in accordance to CSCs play also major roles in tumor metastasis, recurrence and drug resistance.⁹

The metabolic state of CSCs is significantly attributing to therapy resistance. CSCs contrary to CCs have lower bioenergetic metabolism, lower ROS level, and lower capacity to detoxify RO/NS a factor for chemo- or radioresistance,^10,13 whereas mitochondrial functions (ATP production, Δψm) can be decreased. The above manages their metabolic reprogramming, enables them to sustain self-renewal, promote antioxidant defense mechanisms and adapt to hypoxia through glycolytic- oriented metabolism.^13,14 Targeting the redox state of pathogenic versus nonpathogenic cells may represent a challenging solution, confirming or denying that drug resistance in CCs can be similar to CSCs.

Further analysis of mitochondrial function as a therapeutic target to induce cell death in CSCs is greatly significant, as the roles of mitochondria may make CSCs more prone to anti-cancer treatment and apoptosis, and subsequently help develop novel planning for cancer treatment, including through combined therapy with specific mitochondrial-targeting drugs. Undoubtedly, the ineffectiveness of classical anti-cancer therapies has been attributed to the cancer stem cells (CSCs), whereas effective anti-cancer drugs should target CSCs in addition to non-CSCs via combination therapy with specific mitochondrial-targeting drugs. Mitochondria- targeted therapy may initiate new therapy plans regarding relapsed and refractory cancer supporting further the notion that combined treatment with mitochondrial- targeted and anti-cancer drugs could induce the death of both CSCs and general CSs and can become a novel cancer therapy.

Abbreviations

TKIs, Tyrosine Kinase Inhibitors; VEGFR, Vascular Endothelial Growth Factor Receptor; MTC, Medullary Thyroid Carcinoma; Δψm, Mitochondrial membrane potential; CCs, Cancer cells; CSCs, Cancer stem cells; OXPHOS, oxidative phosphorylation; EMT, Epithelial-Mesenchymal Transition; RO/NS, Reactive oxygen/nitrogen species; ROS, Reactive oxygen species

Author contributions

ECT wrote the manuscript, RD, GK, EP and AT revised the manuscript. All authors read and approved of the manuscript.

This is a review of a commentary on a manuscript, “Vandetanib and cabozantinib potentiate mitochondria-targeted agents to suppress medullary thyroid carcinoma cells” recently published by Dmytro Starenki et al. The commentary provides clinical background on vandetanib and cabozantinib, highlighting toxicity and limited efficacy as single agents.

The reviews then suggest that mitochondria-targeted agents are more effective compared to other agents in triggering cancer stem cell (CSC) and cancer cell apoptosis. The metabolic state of CSCs significantly contributes to therapy resistance. The authors state that “effective anti-cancer drugs should target CSCs in addition to non-CSCs via combination therapy with specific mitochondrial-targeting drugs.”

This may be true, and suggests further avenues of investigation, but it would be useful to know how the reviewed manuscript contributes to our knowledge of cancer stem cells or mitochondrial targeted therapies.

We thank the Reviewer for his/her valuable comment.

Starenki D and colleagues address a potentially important aspect of MTC resistance. They stated that vandetanib and cabozatinib can affect mitochondrial activity and bioenergetics in MTC cells. Mitochondria are dysregulated in cancer and increased tumor cell dependency on mitochondria can provide an opportunity to design a novel therapeutic strategy, which would be a cabozatinib and vandetanib combination with a mitochondria-targeted agent into targeting MTC suppression and therefore avoiding drug resistance of drug naïve and drug-resistant cells.

However, resistance to a targeted cancer therapy can be invoked by different multiple mechanisms. Processes like the Epithelial-Mesenchymal Transition (EMT) in accordance to CSCs play also major roles in tumor metastasis, recurrence and drug resistance as highlighted in our commentary. Indeed, the metabolic state of CSCs is significantly attributing to therapy resistance, managing their metabolic reprogramming, enabling them to sustain self-renewal, promoting antioxidant defense mechanisms and adapt to hypoxia through glycolytic-oriented metabolism.

In order to develop efficient treatments that can induce a long-lasting clinical response preventing tumor relapse it is important to develop drugs that can specifically target and eliminate CSCs plus non-CSCs. Recent identification of surface markers and understanding of molecular features associated with the CSC phenotype has improved the design of effective treatments (which goes beyond the presentation of this word-limited commentary). Furthermore, targeting surface biomarkers, signaling pathways that regulate CSCs self-renewal and differentiation involved in apoptosis and resistance, microenvironmental signals that sustain CSCs growth, manipulation of miRNA expression, and induction of CSCs apoptosis and differentiation, with specific aim to hamper CSCs regeneration and cancer relapse, contributes more and more to our knowledge of cancer stem cells as presented in the literature. Some of these agents are under evaluation in preclinical and clinical studies in combination with traditional therapies. Therefore, with our comment we would like to highlight that a novel therapeutic strategy would be the combined therapy using conventional anticancer drugs with CSCs-targeting agents enhanced by mitochondrial targeted therapies, which could offer a promising strategy for management and eradication of different types of cancers.

Further analysis of mitochondrial function as a therapeutic target to induce cell death in CSCs is greatly significant, as the roles of mitochondria may make CSCs more prone to anti-cancer treatment and apoptosis, and subsequently help develop novel planning for cancer treatment, including through combined therapy with specific mitochondrial-targeting drugs. Effective anti-cancer drugs should target CSCs in addition to non-CSCs via combination therapy with specific mitochondrial-targeting drugs. Mitochondria-targeted therapy may initiate new therapy plans regarding relapsed and refractory cancer supporting further the notion that combined treatment with mitochondrial-targeted and anti-cancer drugs could induce the death of both CSCs and general CSs and can become a novel cancer therapy.

Finally, we would like to highlight that the recent progress in developing anti-cancer stem cell strategies is based on improved understanding of CSCs properties and molecular features. These novel therapeutic systems are designed with the aim of eradicating CSCs, by targeting surface specific markers and altering signaling pathways or mechanisms involved in CSCs maintenance and drug resistance, and also to disturb microenvironmental signals that sustain CSCs growth, with specific aim of impede CSCs regeneration and cancer relapse which must be combined focused on further analysis of mitochondrial function as a therapeutic target to induce cell death in CSCs.

In addition, some minor comments and edits are suggested below as tracked changes which may be accepted as appropriate:

We thank the Reviewer for his valuable corrections. We have accepted all changes and minor comments as suggested.