Intratumoural tigilanol tiglate in the multicentre treatment of equine sarcoids and cutaneous melanomas.

Background: Intralesional chemotherapeutic administration represents an important treatment option for equine cutaneous neoplasia. Tigilanol-tiglate (TT), a novel molecule extracted from Fontainea picrosperma, an Australian rainforest plant, is registered for intratumoural treatment of canine MCT, leading to rapid oncosis and tumour slough. Evidence from horses is limited but suggests that efficacy may be similar.

Objectives: To evaluate the response to intratumoural TT treatment in horses with sarcoids (fibroblastic/nodular) and cutaneous melanomas.

Study design: Two noncontrolled prospective multicentre clinical trials, one for each of sarcoids and melanomas.

Methods: Cases were enrolled across multiple sites and treated by the same site-specific clinician with intralesional TT (sarcoids: 0.35 mg/cm³; melanomas: 0.2 mg/cm³ of tumour volume - T_vol; max dose 2 mg). Quantitative (T_vol regression) and qualitative outcomes (likely tumour free (LTF) per expert opinion) were recorded, and potential determinants of efficacy were assessed using random effects logistic models. A full clinical response was complete T_vol regression and a LTF treatment site.

Results: Forty-one sarcoids and 97 melanomas were enrolled and treated. 73/74% of treated sarcoids/melanomas showed complete T_vol regression. 64/61% (sarcoids/melanomas) showed a full clinical response at medians of 546/247 days post final treatment. For both tumour types, this response was dependent on initial tumour volume (P_sarcoids = 0.006; P_melanomas <0.001). The predicted probability of a full clinical response was 6 times greater for initially small sarcoids (T_vol = 1 cm³) than for the maximum study volume (T_vol = 6 cm³). For melanomas in the perineal region, this was 11 times greater for T_vol ≤0.3 cm³ than for tumours ≥2.0 cm³. For melanomas, tumour location further affected treatment efficacy  = 0.005). In total, 5 adverse events were reported.

Main limitations: Lack of treatment control and histologic/biomolecular follow-up data.

Conclusions: The observed therapeutic efficacy of TT supports clinical use as well as early interventions in horses. Successful use necessitates knowledge of the drug's mode of action and management of associated local site responses.