Metanephrine is a metabolite of epinephrine produced by the action of catechol-O-methyl transferase on epinephrine. [1] Determination of plasma or urinary metanephrine is considered the principal test for the clinical diagnosis of pheochromocytoma, an adrenal medullary neuroendocrine tumor.[2]

Reference:
[1]. Kopin, I.J., Axelrod, J., and Gordon, E. The metabolic fate of H3-epinephrine and C14-metanephrine in the rat. The Journal of Biological Chemisty 236, 2109-2136 (1961).
[2]. Wolthers, B.G., Kema, I.P., Volmer, M., et al. Evaluation of urinary metanephrine and normetanephrine enzyme immunoassay (ELISA) kits by comparison with isotope dilution mass spectrometry. Clinical Chemistry 43, 114-120 (1997).