The heparan sulfate mimetic PI-88 is a potent inhibitor of HIV-1 independently of its anti-heparanase activity.

Heparanase-1 (HPSE) is a multifunctional enzyme with well-established pro-carcinogenic and pro-viral activities, implicated in the life cycles of several viruses, including Herpes simplex virus types 1 and 2, and Hepatitis C virus. However, its potential contribution to Human immunodeficiency virus type 1 (HIV-1) infection has remained unexplored. Here, we investigated the role of HPSE in HIV-1 replication using complementary pharmacological and genetic approaches. The clinically advanced HPSE inhibitor PI-88 (muparfostat) exhibited potent anti-HIV-1 activity at nanomolar concentrations in cell lines and primary cells, and significantly reduced viral infection in chronically infected cells as well as in humanized mice. Mechanistic studies revealed that PI-88 acts primarily during viral attachment, likely through steric blockade of the CD4-binding pocket on gp120 - an entry-inhibitory effect that is reversible. Surprisingly, this robust antiviral effect of PI-88 was independent of its anti-HPSE activity. CRISPR/Cas9-mediated HPSE depletion enhanced HIV-1 infection, whereas exogenous overexpression of HPSE rather reduced it, which is in contrast with previous reports on other viruses, where HPSE downregulation inhibits infection. Taken together, our results identify PI-88 as a strong inhibitor of HIV-1 with a mechanism unrelated to its anti-HPSE activity, and reveal an unanticipated antiviral function for HPSE itself. Given that PI-88 has successfully completed phase II/III clinical trials as an anti-cancer agent, our findings support its potential as a candidate for drug repurposing and as a novel complementary therapeutic strategy for HIV-1.