At the 400 mg dosage, there's a concern for CD4+ T cells dropping below the crucial level (100-200 cells/mm^3) of preventing PML, a CNS infection caused by the JC virus.

"Although most CNS opportunistic infections are almost wholly prevented when CD4 T-lymphocyte (CD4 cell) counts are maintained above 100 to 200 cells/mm3 PML can still sometimes occur in patiens treated effectively with ART."

https://aidsinfo.nih.gov/guidelines/html/4/ad...46/pml-jcv

Genentech had to withdraw its psoriasis drug, Raptiva, from the US market due to a potential risk of PML in 2009.

"Today, Genentech, the manufacturer of the psoriasis drug Raptiva (efalizumab), announced that it has begun a voluntary, phased withdrawal of the product from the U.S. market. The company is taking this action because of a potential risk to patients of developing progressive multifocal leukoencephalopathy (PML), a rare, serious, progressive neurologic disease caused by a virus that affects the central nervous system."

https://www.fda.gov/Drugs/DrugSafety/Postmark...143347.htm

Good news is that Prurisol reduces CD4+ T cells less than Raptiva. As F1ash and biodoc pointed out from the preclinical study (link no longer exists on the IPIX website),

"Treatment with Prurisol caused less reduction in CD4+ lymphocyte counts than did efalizumab."

Efalizumab is Raptiva. This is probably why the 400 mg arm has 1/3 of the sample size as the 300 mg arm (27 vs. 81 patients).