Cyclosporin Implants: When and Why?

Cyclosporin (CsA) implants (CsA ocular sustained release medication devices) are used in the treatment of equine recurrent uveitis (ERU). CsA is an immunosuppressive drug which blocks interleukin-2 transcription through the inhibition of calcineurin and impairs proliferation of activated T-helper and T-cytotoxic cells. The devices are surgically implanted into the suprachoroidal space (between the ciliary body and the sclera) and allow a constant therapeutic level of CsA to be delivered to the site of action within the eye (the uveal tract), effectively bypassing some of the blood ocular barriers, and eliminating the need to rely on owner compliance in treating the horse. The release rates of CsA allow high concentrations to be reached within the eye without any systemic toxicity. The theoretical depletion of CsA from the implant is 3.2 years.

The aetiopathogenesis of the ERU disease complex is multifactorial with the horses’ genetic makeup, immune system and environmental factors, in particular the exposure toLeptospira spp., all having important roles. Although debate continues over the importance of various factors in initiating ERU, there is an overall consensus that ERU is immune-mediated. More specifically, ERU is defined as a T-helper type 1 (Th1)-mediated disease. This underlying T cell-mediated disease process explains why CsA, which targets T cell activation, is effective in preventing the reactivation of the ocular inflammation in ERU patients. It is therefore important that ERU is accurately diagnosed, as the CsA implant may not be effective for infectious or other non-ERU causes of uveitis. [...]