Tin‐Protoporphyrin Prevents Experimental Superficial Siderosis in Rabbits

Superficial siderosis of the human central nervous system is caused by small continuous or recurrent subarachnoid hemorrhages that lead to the destructive deposition of hemosiderin. The excessive tissue iron derives from heme that is oxidized in a rate-limiting step by the enzyme heme oxygenase (HO), and especially the inducible form, HO-1. We postulated that competitive inhibition of HO by tin-protoporphyrin IX (SnPP) could prevent experimental superficial siderosis. Since synthetic metalloporphyrins do not cross the blood-brain barrier, SnPP was delivered directly into the cisterna magna. Rabbits received weekly intracisternal injections of washed autologous red blood cells (RBC) over a period of 1 to 16 wk. In companion experiments, SnPP was added to the suspension of RBC, or SnPP was injected without RBC. All injections caused increased HO-1 immunoreactivity in the Bergmann glia of the cerebellar cortex and in superficial astrocytes of the piriform cortex. The injections of RBC or RBC with added SnPP also generated a vigorous microglial response. The metalloporphyrin entered the tissue in inhibitory amounts and greatly reduced the accumulation of histochemically detectable iron. It did not alter the microglial response. The observations allowed the conclusion that SnPP suppressed heme oxidation but did not affect other steps in the pathogenesis of superficial siderosis.