Polymyxin B: What is the evidence for its use in the surgical colic patient?

Optimal supportive medical management of the surgical colic patient is an area of debate. A variety of therapies are cited in the literature but there is relatively little evidence about the effectiveness of some therapies in the clinical setting. For many horse owners, economic factors are a major issue when considering undertaking colic surgery and continuing with hospital care, of which medical therapies may constitute a significant proportion of these costs. A continued trend towards euthanasia at / without surgery and decreased proportion of horses going to surgery has been reported in two hospital populations in the UK and USA [1]. In order to keep colic surgery affordable for many horse owners, it is important to consider the cost- benefit of use of the various medical therapies proposed. In addition any potential side-effects and drug interactions must be considered where multiple medical therapies are beingutilised.

The Systemic Inflammatory Response Syndrome (SIRS), what some refer to as clinical endotoxaemia, remains a challenge for equine clinicians in the perioperative care of some surgical colic cases [2,3]. Polymyxin B is commonly cited as a therapy for managing these cases but what is the evidence for its use in horses undergoing surgery for colic?

Polymyxin B (PMB) and research studies

The cationic antimicrobial Polymyxin B (PMB) forms a stable 1:1 stochiometric complex with lipopolysaccharide binding protein (LPS) by binding to the lipid A component thereby neutralising it. PMB has shown potential as a therapeutic agent to treat endotoxaemia in both in-vitro and in-vivo horse models [4-7]. PMB administration prior to and immediately after LPS administration in healthy horses reduced the deleterious systemic effects of LPS [5]. Nephrotoxic and neurotoxic side effects were not seen in healthy horses at a dosage of 1mg(6,000 IU) / kg every 8 hours for up to 5 treatments [7]. However, in azotaemic patients, it is recommended that PMB should be used judiciously with careful monitoring of renal function in patients [7]. [...]