Isoxanthopterin interferes with RNA and DNA synthesis.

Isoxanthopterin, a natural intermediate in the pteridine pathway, plays critical roles in pigmentation and cell division.

In vitro: Isoxanthopterin was identified as a pteridine of wide natural occurrence and was reported to exerts an inhibitory action on the synthesis of ribosomal RNA, possibly soluble RNA and on DNA in developing eggs of the milkweed bug Oncopeltus fasciatus. Such effects were measured by partially incubating dechorionated eggs in a solution containing isoxanthopterin and radioactive uridine or thymidine, and then testing the incorporation of the isotope into ribosomal RNA or DNA. The detailed mechanism of this inhibition had been discussed, and the tentative conclusion was agreed that it was due to an interaction between the pteridine and the nucleic acid (most probably DNA) [1].

In vivo: Previous animal study showed that rats with inherited retinal dystrophy appeared to differ from healthy rats by an elevated isoxanthopterin excretion in urine. RCS rats responded to feeding of monapterin with an increase of isoxanthopterin excretion. This phenomenon supported the idea that the RCS rat had an increased xanthinoxidase activity compared with healthy rats [2].

Clinical trial: So far, no clinical study has been conducted.

References:
[1] Lagowski, J. M. and Forrest, H.S. Interaction in vitro between isoxanthopterin and DNA. Proceedings of the National Academy of Sciences of the United States of America 58(4), 1541-1547 (1967).
[2] G.  Cremer-Bartels, H. Gerding, L. Hanneken and K. Krause. Isoxanthopterin Excretion of Rats with Inherited Retinal Dystrophy. Pteridines Vol. 2, 1990, pp. 103 -105.