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Mitochondria were the first intracellular organelles to be associated with calcium handling [29] and in 2004, the mitochondrial calcium uniporter located in the inner mitochondrial membrane was identified as the key means through which mitochondria regulate calcium uptake and accumulation in the mitochondrial matrix [30] (Figure 12.1). Pathologically, calcium overload within the mitochondrial matrix leads to opening of the mitochondrial permeability transition pore (MPTP) across the inner mitochondrial membrane, allowing unregulated entry and exit of particles up to 1.5 kDa into and out of the mitochondrial matrix [13]. This results in the loss of inner mitochondrial membrane potential, diminished ATP production, mitochondrial swelling and rupture of the outer mitochondrial membrane followed by necrotic cell death [31–34]. Mitochondrial dysfunction as a result of intracellular calcium overload induced by toxins that include bile acids and ethanol metabolites has become established as a key pathogenic mechanism in acute pancreatitis [9,13,35,36]. MPTP opening is physiological in low‐conductance mode, releasing calcium and reactive oxygen species to match metabolism with workload, but pathological in high‐conductance mode compromising ATP production and inducing cell death [33,34]; both functions are regulated by the mitochondrial matrix protein peptidyl‐prolyl cis‐trans isomerase (PPI) cyclophilin D [CypD, also known as cyclophilin F (ppif) in mice located on chromosome 14] [37]. Little is known about the physiological role of CypD; CypD knockout (ppif ^–/–) mice are born healthy at the expected Mendelian ratios [37], but display increased anxiety and adult‐onset obesity [38]. Beyond behavioral traits, the lack of significant phenotype suggests that CypD inhibition may carry a low risk of toxicity. The role of CypD and MPTP opening in disease derives from studies of ischemia–reperfusion injury in the heart, brain, lung and kidney, muscular dystrophies, neurodegeneration, osteoporosis, and AP [13,31,33,34,39–42].

Figure 12.1 Pancreatic acinar cell focused AP treatment strategies. Physiological acinar cell calcium (Ca²⁺) signaling occurs through muscarinic (M3R) or cholecystokinin (CCK1R) receptors coupled with the second messengers inositol trisphosphate (IP₃) and nicotinic acid adenine dinucleotide phosphate (NAADP) acting on endoplasmic reticulum‐based IP₃ receptors (IP₃R) and ryanodine receptors (RyR), respectively, resulting in store‐operated Ca²⁺ entry (SOCE). Abnormal Ca²⁺ signaling in the pancreatic acinar cell initiated by pancreatitis toxins (e.g. bile acids, fatty acid ethyl esters, hyperstimulation) causes injury dependent on continued SOCE via ORAI channels. As a consequence mitochondria are overloaded with Ca²⁺ thought to be via the mitochondrial calcium uniporter (MCU), leading to induction of the mitochondrial permeability transition pore (MPTP), allowing the passage of solutes <1500 kDa across the mitochondrial membrane with subsequent loss of the mitochondrial membrane potential and reduction in ATP production, required to clear Ca²⁺ through sarcoplasmic/endoplasmic reticulum Ca²⁺‐ATPase (SERCA) and plasma membrane Ca²⁺‐ATPase (PMCA) pumps and thus protect the cell. Protective acinar cell strategies that have shown significant beneficial effects in a variety of experimental pancreatitis models focus on the prevention of calcium overload, the reduction of mitochondrial injury, the modulation of autophagy, and serine protease or serine protein kinase inhibition.

Cyclosporin A (CsA), a macrocyclic oligopeptide and nonspecific inhibitor of cyclophilins, has played a significant role in studying MPTP in in vitro and in vivo experimental models of different diseases. Its large size, poor solubility, and immunosuppressive nature (from interaction with calcineurin [43]) are properties that hinder its use in AP. As a result of initial promising preclinical findings, some non‐immunosuppressive analogs of CsA have also been synthesized including Debiopharm’s DEB025 (alisporovir) and Novartis’s NIM‐811 [39]. Our research has demonstrated MPTP to be a valid target for AP treatment by inhibiting CypD genetically (ppif ^–/–) and pharmacologically using CsA, DEB025 and TRO40303 (Trophos) [13]. DEB025 proved effective in maintaining membrane potential and inhibiting necrosis in freshly isolated murine and human pancreatic acinar cells exposed to pancreatitis toxins. Moreover, DEB025 significantly reduced all the biochemical and histological parameters in five different experimental models of AP. Common to all ciclosporin analogs synthesized thus far, DEB025 also inhibits other cyclophilins; as a result of its inhibition of cyclophilin A, it was in clinical development by Novartis as a treatment for hepatitis C. This development has been abandoned, however, and at present DEB025 is not in clinical development. It remains unclear whether nonspecific cyclophilin inhibition will benefit or detract from the treatment of AP, but the high amino acid sequence homology between cyclophilins means the development of novel molecule inhibitors specific to CypD remains a challenge [44]. Non‐immunosuppressive sanglifehrin analogs, which are orally bioavailable and lighter in molecular weight compared to CsA, have also been developed [45] but have not been tested in experimental AP. We are also developing CypD inhibitors, as this strategy holds significant promise for future progression to early phase trials [41,46–48]. As we and others have demonstrated a reduction of ATP from mitochondrial dysfunction to be an important contributor to AP pathogenesis [9,13,36], we look forward to the results of the current GOULASH trial comparing high‐ versus low‐energy nutrition in the early phase of AP [49].