Introduction – Ischaemic Heart Disease is the leading cause of death worldwide1. Gold standard therapy is reperfusion of the tissue at risk. However, the act of reperfusion is associated with lethal reperfusion injury – death of cells only reversibly injured by the preceding ischaemic insult2. Current cardioprotective measures, ischaemic pre and post-conditioning (IPC & iPOC), reduce opening of the mitochondrial permeability transition pore (mPTP) following reperfusion – a key mediator of both necrotic and apoptotic cell death3. Matrix Metalloproteinase-2 and -9 are located in the mammalian heart4, and inhibition of these MMPs reduces infarct size following ischaemia-reperfusion5. The mechanism of cardioprotection by MMP inhibition is not known. Aims – This investigation aimed to identify whether ilomastat, an MMP inhibitor, works by a mechanism other than inhibition of mPTP opening. We hypothesized that cardioprotection is independent of mPTP inhibition, the end-effector of iPOC. Methods & Results – Experiments using animals were performed in accordance with the UK Home Office Guide on the Operation of Animal Act. The mice were anaesthetized by intraperitoneal injection with a combination of ketamine, xylazine and atropine (0.01 ml/g). Efficacy was determined by pedal pain withdrawal reflex before thoracotomy was undertaken. In ex-vivo and in-vivo mouse hearts, we investigated whether the MMP inhibitor, ilomostat (0.25 mmol/L), at reperfusion, could engender protection in the absence of cyclophillin-D (CyPD), an initiator of mPTP opening, against injurious ischaemia/reperfusion. Ilomastat attenuated infarct size in wild type (WT) animals (37±2.8 to 22±4.3%, equivalent to iPOC, 27±2.1%, p<0.05). CyPD knockout (KO) hearts had smaller infarcts compared to their WT brethren (28±4.2%) and iPOC failed to protect, indicative of a pre-protected phenotype, yet ilomastat significantly attenuated infarct size in these hearts (11±3.0%, p<0.001). Furthermore, ilomastat was additive to the protection seen following iPOC in WT, restoring protection even after prolonged 50min ischaemia (49±7.8 to 31±2.8%, p<0.05). Moreover, ilomastat had no inducing effect on the reperfusion injury salvage kinase signalling (RISK) cascade (requisite for IPC and iPOC protection) and, unlike cyclosporine, had no impact upon mPTP opening, indicating no interaction with CyPD/mPTP in isolated cardiomyocytes. Conclusions 1. MMP inhibition with ilomastat attenuates infarct size, independently of the mPTP and the RISK cascade 2. Ilomastat protection is additive to mPTP inhibition (and also to ischaemic postconditioning -[data not shown]) 3. MMP inhibition remains protective even after more prolonged durations of ischaemia 4. Cardioprotection independent of mPTP manipulation suggests potential to further limit infarct size when ilomastat is combined with mPTP inhibition setting of acute coronary syndrome