20 Feb 2024
While surgical excision of tumours in equids is an ideal treatment in many cases, in others intratumoural injection of a chemotherapeutic agent – and short and intense electrical pulses applied to the local tissues – may be a better option…
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Electrochemotherapy (ECT) is a delivery mechanism that enables enhanced local intracellular penetration of certain chemotherapy drugs. It increases their cytotoxic therapeutic effect while minimising the dose required, and accordingly the likelihood of morbidities associated with systemic toxicity.
After intratumoural injection of the chemotherapeutic agent, short and intense electrical pulses are applied to the local tissues, leading to transient permeabilisation of the cell membrane and delivery of the drug to its targets within the cell interior.
With proven efficacy displayed in the treatment of various tumour types in humans (Tozon et al, 2015), ECT has been gaining popularity as a minimally invasive, local therapy for the curative or palliative treatment of cutaneous and subcutaneous solid tumours in dogs, cats and horses.
The skin is one of the most common sites to detect tumours in equids (Spugnini et al, 2016a), and 95% of these cutaneous neoplasms are of three types: sarcoid, melanoma or squamous cell carcinoma (SCC; Spugnini et al, 2021). Theoretically, surgical excision of these solid tumours with wide margins would be the ideal treatment, as, while locally aggressive, distant metastases are uncommon with sarcoids and pose a small risk with SCC and melanoma. However, their tendency to occur as a large number of individual lesions, have an unpredictable clinical course, be deeply invasive and occur around important structures and in anatomical locations where there is little free tissue for wide resection limits the ability of excision to be curative.
For sarcoids in particular, numerous other treatment options such as cryosurgery, laser surgery, radiotherapy, photodynamic therapy, immunotherapy with BCG and chemotherapy are available, but no universal treatment is successful for all lesions in all locations. Additionally, melanomas are poorly chemoresponsive neoplasms, which means that high cytotoxic drug concentrations are needed for an effect to be realised (Scacco et al, 2013).
Although radiation brachytherapy is often highly effective as a treatment for sarcoids, in reality the need for specialist facilities and equipment limits its use (Tamzali et al, 2012), with no high dose rate currently available in the UK following the closure of the AHT.
In recent years, intratumoural chemotherapy with cisplatin has become a popular treatment for sarcoids. Cisplatin is also one of the few chemotherapeutic agents that shows efficacy against melanomas, but side effects limit its systemic use (Scacco et al, 2013) and quick metabolism of the aqueous preparation means that use of a drug carrier is essential to ensure adequate exposure of neoplastic cells (Tamzali et al, 2012).
Sesame oil and biodegradable beads have been used as effective drug carriers for cisplatin and are easy to insert into the tumour, but release of drug for up to three months post-insertion represents a significant carcinogenic risk to both the vet and those caring for the horse (Tamzali et al, 2012).
Accordingly, and with such seemingly benign lesions potentially leading to euthanasia of horses due to compromised welfare or an inability to perform their intended purpose, a clear need exists for a safe, feasible and effective treatment option: something ECT may be a potential answer to.
The cell membrane acts as a barrier, preventing or limiting intracellular penetration of many chemotherapeutic drugs and subsequent contact with their internal targets. ECT effectively acts as a mechanism to augment intracellular uptake of chemotherapy drugs through a localised, transient increase in permeability of the cell membrane, and thus results in enhanced toxicity due to increased intracellular drug concentration.
Bleomycin and cisplatin are the drugs that have proven efficacy when used in combination with ECT (Tellado et al, 2022). After intratumoural injection of these drugs, an electric field is induced in the treatment area through application of electrodes. As cells in the treatment area are exposed to this external electric field, a transmembrane potential, which is dependent on the shape and size of the cell, is generated (Cemazar et al, 2008). This leads to an electrically mediated de-stabilisation of the cell membrane through a rearrangement of the lipid bilayer (Spugnini et al, 2016b) and formation of transient transmembrane defects, known as electropores (Cemazar et al, 2008).
As long as the correct electrical parameters are selected, these membrane alterations are temporary and will re-seal after a few minutes (Cemazar et al, 2008), but critically allows entry of normally non-permanent lipophobic drugs. It has been reported that through this electropermeabilisation of cells, the cytotoxicity of bleomycin and cisplatin is increased several 100-fold and 70-fold, respectively (Cemazar et al, 2008).
Neoplastic cells are thought to have more unstable cell membranes than that of healthy cells (Spugnini et al, 2016b) engendering some therapeutic selectivity, and the localised increased intracellular drug penetration allows use of lower drug doses, reducing the likelihood of systemic adverse side effects and enabling safer use of drugs with a narrow therapeutic index.
Although increased cytotoxicity through electropermeabilisation is thought to be the main mechanism of action for ECT, two others have been suggested: vascular lock and an immune mechanism. Application of an electric pulse to cells also causes localised vascular disruption and a transient hypoperfusion (Probst et al, 2018; Tellado et al, 2022).
Known as vascular lock, this reduction in blood flow has an additional therapeutic benefit through increasing drug retention within the tumour and providing immediate haemostasis when resecting highly haemorrhagic tissues (Probst et al, 2018; Tellado et al, 2022). Additionally, local cellular damage induces release of tumour antigens into the local microenvironment and bloodstream (Probst et al, 2018; Tozon et al, 2016), potentially activating a tumour antigen-directed immune response against any distant metastases to realise a more systemic effect (Probst et al, 2018).
As well as with chemotherapeutic agents, ECT can also be used in combination with calcium, where it is more commonly known as calcium electroporation. Calcium is a key second messenger involved in many cellular processes, and ordinarily intracellular calcium levels are tightly regulated at a very low level. Accordingly, the enhanced membrane permeabilisation elicited by ECT leads to an intracellular calcium overload and controlled cell death via apoptosis (Galant et al, 2019). This has the advantage that no cytotoxic drugs are required, increasing both patient and personnel safety.
So far in veterinary oncology, ECT has mainly been used for the treatment of sarcoids and melanomas, where it has shown comparable efficacy to other standard local treatments (Cemazar et al, 2008; Tozon et al, 2015).
It has also been used in more specific cases, such as advanced squamous cell carcinoma of the hoof where bleomycin was administered via an isolated limb perfusion followed by administration of electric pulses (Spugnini et al, 2017). It has additionally been used as an adjuvant treatment for an incompletely excised, recurring penile fibrosarcoma (Spugnini et al, 2016a).
ECT has proven to be a safe and effective treatment, with adverse reactions limited to localised oedema and erythema at the tumour site, lasting for several days after treatment. It has been used both as a sole treatment option and an adjunctive therapy following surgical de-bulking of larger tumours, and equally against primary or recurrent lesions.
As many individual factors must be taken into account for the effective treatment of each tumour, no generic protocol can be advised (Probst et al, 2018), but the following outlines a general description of the procedure.
A key point to note is that ECT must be carried out under general anaesthesia (GA), as application of the electric current leads to muscle contractions that, although usually limited to the local cutaneous muscles, can result in stronger movements if the treatment area is close to a nerve plexus (Galant et al, 2019), such as in the axilla or groin.
This is currently a significant disadvantage of ECT, which, in our clinic at least, limits use due to cost and feasibility. Although use has been reported under a romifidine drip and caudal epidural (Scacco et al, 2013), we would only advocate administration under GA for safety reasons.
The volume of the tumour must first be determined following measurement with callipers, to allow calculation of an appropriate chemotherapy drug dose. At the authors’ clinic, a dose of 0.3mg/cm3 of cisplatin is normally used, and the dose split in multiple injections spaced throughout the tumour tissue to ensure an even drug distribution (Tamzali et al, 2012) and with the aim of saturating the area.
The electric pulses must then be quickly applied while the maximum amount of drug is present within the tumour tissues (Cemazar et al, 2008). Depending on the anatomical location of the mass to be treated, needle or plate electrodes can be used: needle electrodes allowing the electric pulse to penetrate to a greater depth, but with a more heterogenous distribution, and plate electrodes allowing a more even distribution over superficial surface tumours (Cemazar et al, 2008). In some cases, one treatment session can be enough to ensure good results, but repeat treatments are advocated for more extensive or recurrent lesions (Tellado et al, 2022).
At the authors’ clinic, the standard treatment protocol would be three sessions of ECT carried out at day 1, 14 and 28; usually selected for the more aggressive and recurrent lesions. Where large tumour nodule size dictates the need for primary resection prior to application of ECT, laser excision has been used over sharp excision due to its accuracy, haemostatic properties and the ability to easily dissect between fascial planes with die back beyond the cut margins due to deeper vaporisation of cells.
The protocol has been mainly used in sarcoids with undefined margins, or at anatomical locations were tumours as prone to infiltrative growth, but it is difficult to obtain margins with wide resection, such as groin, axilla, ventral body wall, periocular, facial and distal limb. ECT has also been used as an adjunctive single treatment session when concerns arise during laser surgery about the margins that have been achieved.
Unfortunately, the referral population makes long-term follow-up difficult, but clinical impression is that of an easy to perform additional treatment that can improve outcome in complex cases, with minimal side effects to the horse.
A lot is still unknown about ECT in terms of the best electric pulse voltage, drugs and treatment combinations to use in horses, with protocols often being extrapolated from standard procedures used in the human clinical setting, and resulting in reduced effectiveness in our patients (Tellado et al, 2022).
However, ECT has shown early promise and has the potential to be highly effective in veterinary oncology, opening the way for drugs previously eliminated, due to high toxicity, to now be used (Spugnini et al, 2016b). It must be remembered, though, that ECT will not improve uptake of all chemotherapy drugs (Spugnini et al, 2016b), depending on their molecular structure, and its use is currently only recommended in combination with bleomycin, cisplatin or calcium.
