A method of administration of a thiol-based chemoprotectant agent including NAC (N-acetylcysteine) and STS (sodium thiosulfate) that markedly affects biodistribution and protects against injury from diagnostic or therapeutic intra-arterial procedures. A method for treating or mitigating the side effects of cytotoxic cancer therapy for tumors located in the head or neck and brain tumors. The thiol-based chemoprotectant agent is administered intra-arterially with rapid and first pass uptake in organs and tissues other than the liver.