Electrochemotherapy with cisplatin enhances local control after surgical ablation of fibrosarcoma in cats: an approach to improve the therapeutic index of highly toxic chemotherapy drugs

Cancer is among the major causes of death in the human population, being responsible for millions of deaths each year [1, 2]. Systemic chemotherapy is the most commonly used therapeutic strategy, although considerable limitations exist since conventional chemotherapy often involves pulsatile administration schedules using maximum tolerated doses (MTDs) of cytotoxic drugs. The long break periods between therapies not only allow recovery from various toxicities, especially myelosuppression, but also provide an opportunity, unfortunately, for the drug-treated tumors to recover as well [3‐8]. The therapeutic index (also known as therapeutic ratio), is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes death (in animal studies) or toxicity (in human studies). Some drugs have such a narrow therapeutic index to be associated with significant systemic side effects, including gastro-intestinal toxicity, cardiac toxicity and bone marrow depletion, that could result, among other complications, in hemorrhage and sepsis [9‐11]. Furthermore, systemic chemotherapy is often not efficient in delivering drugs to target sites at therapeutic concentrations, and maintaining adequate drug levels within tumors is a challenge [2‐14]. Chemotherapeutic delivery to solid tumors systemically involves several limiting factors, including the role of the i) drug transport along the blood circulatory system to tissues (including also the issue of plasma binding proteins), ii) interstitial space, iii) drug removal by capillaries and, last but not least, iv) tissue structure and composition with respect to the drug distribution [1, 12‐14]. As a result, only a fraction of the administered dose reaches tumor cells, which dramatically hinders tumor targeting, prevents effective therapy and increases toxicity to healthy tissue [1, 12‐14]. Considering that more than 85% of human cancers are solid tumors [8], several strategies have been adopted to overcome these problems, including intra-arterial chemotherapy [15], chemotherapy impregnated implants [16‐18] and polymeric drug delivery systems [8]. Despite the continuous investigation of alternate routes for improved systemic chemotherapeutic delivery there has been minimum therapeutic gain [8, 15‐18], therefore localized delivery has gained increased attention in cancer therapy. This strategy aims at maintaining low systemic drug levels while ensuring therapeutic levels at target sites, thus aiming to deliver a much more efficient drug therapy [8, 14]. Selective drug retention at target sites, with consequent decreased systemic drug exposure, is an important issue, since two therapeutic goals are minimizing toxicity and maximizing efficacy [19]. This selectiveness can be accomplished through electrochemotherapy (ECT) a cancer treatment that involves the application of permeabilizig electric pulses having appropriate waveforms coupled with the local or systemic administration of chemotherapy agents [20, 21]. Over the past years, several ECT studies have been conducted on companion animals affected by advanced spontaneous tumors, obtaining high response rates, while surgery alone, especially for rapidly growing tumors such as feline sarcomas results in control times ranging from 60 to 270 days [22‐25]. Cisplatin is a chemotherapy agent widely adopted in veterinary oncology that cannot be administered systemically to cats since it induces severe pulmonary toxicoses including dyspnea, hydrothorax, pulmonary edema, mediastinal edema and death [26, 27]. Aim of this investigation is the evaluation of toxicoses and efficacy of local cisplatin as agent for adjuvant ECT in a spontaneous feline sarcoma model.

The Regina Elena Cancer Institute Ethical Committee approved this study that was performed according to the Italian Law (116/92). Seventy-eight privately owned cats with histopathologically confirmed incompletely excised or recurring soft tissue sarcoma (STS) were entered in the modified phase II study. Evidence of recurrence was demonstrated upon the diagnostic histopathology analysis of bioptic specimens excised by the referring veterinarians. Previous informed consent was obtained from the owners. In order to be enrolled in the study, patients had to fulfill the following criteria:

Staging process included a thorough anamnesis, histopathologic assessment of surgical margins, physical examination, complete blood cell count (CBC), serum biochemistry profile, urinalysis thoracic radiographs (three projections), and abdominal ultrasonography. In order to confirm the diagnoses, histological examination of the biopsies was performed following standard protocols, using Hematoxylin/Eosin and Hematoxylin/Van Gieson stainings. Patients receiving adjuvant ECT were matched with a cohort of cats treated with surgery alone, having the owners declined to pursue other treatments than surgery.

Sixty-four cats entered the electrochemotherapy study over a 5 years period and their response has been matched with that of 14 cats treated with surgery alone.

Cancer is one of the most difficult health challenges of our time. In addition to the currently available conventional therapeutic modalities such as chemotherapy, radiotherapy and surgery, there is an urgent need for more effective and less toxic therapies for solid malignancies. Chemotherapy alone shows high toxicity and a low survival rate [6, 7]. In some cases, malignant cells develop resistance to a particular drug and to combat this, a variety of approaches like intra-arterial therapy, induction chemotherapy, immunotherapy, and photodynamic therapy have been employed [8]. Techniques like intra-arterial and induction chemotherapy have showed some improvement in survival rate. Immununotherapy is in the experimental stages, while photodynamic therapy is being clinically applied, but side effects prevented its diffusion. Various treatment strategies have been explored to improve upon current systemic chemotherapy methods. Localized pulse mediated delivery of chemotherapeutics for solid tumors is one approach that has been explored in recent years due to the customizability of the therapy [20, 24]. However, some concerns still need to be addressed. One challenge of localized electrochemotherapy is the lack of control in drug distribution when treating solid tumors. Intravenously delivered chemotherapeutics must overcome barriers including the vasculature wall. Moreover, hypoxic tumor regions distant from vasculature may not be exposed to adequate amounts of drug, thus developing multi-drug resistance. On the other hand, drug penetration following intralesional administration is hindered by the extravascular compartment of tumor tissue, which limits drug diffusion [19, 30]. In this study, to overcome the above limitations, ECT has been used in an adjuvant fashion, after tumor resection, thus limiting its purpose to the eradication of the surviving clonogenic cells and the prevention of tumor recurrence. More importantly, the rapid shifting of cisplatin from the interstitial space to within the tissue, prevented potentially fatal toxicities in this feline model of solid tumor. Furthermore, the increased susceptibility of tumor cells to the permeabilizing pulses, compared to that of normal tissues, allows a semi-selective drug delivery [19‐25], therefore supporting tumor control while side effects are minimized. The results in terms of local control suggest a therapeutic gain when ECT is added to the surgical protocol and compare favorably with the results obtained by our group in a previous study of ECT in cats with soft tissue sarcomas [23]. In that investigation bleomycin has been used since it is the first choice drug for ECT. Cisplatin is a second choice drug for ECT and its systemic administration in cats is not possible due to fatal pulmonary edema [26, 27]. Indeed one of the aims of our study was to evaluate the reintroduction of highly toxic compounds through electroporation in the anticancer protocols. The simple strategy of using electroporation allowed us to greatly increase the therapeutic index of cisplatin in cats, which are very susceptible to this drug. It is conceivable that the increased adoption of this strategy by other investigators will result in the "rescue" of other drugs that otherwise would not be clinically usable, due to excessive systemic toxicity. Also, it is noticeable that only one ECT patient experienced metastatic spread of the primary tumor. The low frequency of this event (1/64, i.e., ~1.6%) compares favorably with what observed in our previous investigation of cats with sarcomas, and more importantly, with the much higher metastatic incidence reported in the literature (depending on the studies it ranges from 10% to 25%) [23].

ECT is an effective strategy to improve the chemotherapy efficacy and the therapeutic index of chemotherapy drugs in the treatment of solid neoplasms. Further studies with a greater number of patients and, possibly, involving multiple institutions should be performed in order to better elucidate the real clinical impact of ECT on STS treatment in feline patients, also in view of a possible translation to humans [24].