There is a significant unmet medical need to develop new therapies aimed at preventing chronic recurrent hypoglycemia in congenital HI, the most common cause of persistent hypoglycemia in children. RZ358 is a human mAb that allosterically attenuates excessive insulin action on target cells. Therefore, RZ358 is ideally suited as a potential therapy for hyperinsulinism, and it is being developed to treat the hypoglycemia associated with diseases such as congenital HI. This is a Phase 2, multicenter, open label clinical study designed to assess the safety and efficacy of four progressively higher doses of RZ358 in separate groups of patients with hyperinsulinemic hypoglycemia due to Congenital HI, not adequately controlled with or without current standard of care. A screening period of up to 5 weeks will evaluate eligibility. Once enrolled, RZ358 will be administered bi-weekly over 8 weeks, and then patients will complete a post-treatment follow-up period of 13 weeks.