PTPN1 and obesity due to melanocortin 4 receptor deficiency: MSI-1436 inhibits PTP1B via a non-competitive allosteric mechanism.6 In Phase 1 and 1b clinical trials as a potential treatment for obesity and diabetes, MSI-1436 was shown to be well tolerated by patients and to induce metabolic changes consistent with PTP1B inhibition.11–13 The doses of MSI-1436 that stimulate tissue regeneration in zebrafish and mice are 5–50-times lower than the maximum well tolerated human dose.