Hunter syndrome attacks the body and — in most patients — the brain. The hardest problem is the blood-brain barrier: standard enzyme therapy treats the body but cannot reach the brain. Every recent advance exists to close that gap. Here is the honest state of the art, ranked by potential to treat the whole disease.
On 25 March 2026, the FDA granted accelerated approval to the first blood-brain-barrier-crossing therapy for MPS II (Denali’s tividenofusp alfa, AVLAYAH) — explicitly for the neurologic disease. A gene therapy (Regenxbio’s RGX-121) is further back after a Complete Response Letter and a safety hold. The field is moving fast — and toward the brain. [source]
Because the neuronopathic (brain) disease is the hardest part — and the part standard therapy can't touch — Operation Hunter prioritizes the brain-reaching science. Rather than betting only on a theoretical idea, a leading strategy is to back or help extend one of the approaches already proven or maturing in research so it fully addresses the central nervous system.
The two that genuinely cross the blood-brain barrier via a transferrin-receptor "Trojan horse" — tividenofusp alfa (AVLAYAH) and pabinafusp alfa (IZCARGO) — plus the two CNS-directed gene therapies in active research — RGX-121 and IDS.ApoEII — are the most promising bases to build on. Details and sources on each page below.
Ranking is our editorial synthesis of cited evidence — by potential to treat the whole disease including the brain — not a competitive or efficacy claim. Each entry links to a detailed, sourced page.
A biomarker is not a cure. Most brain-directed programs are measured by a spinal-fluid sugar marker (heparan sulfate). The intrathecal enzyme trial cut that marker ~72% yet failed to improve cognition — which is exactly why confirmatory cognitive trials are the real test. [source]
Timing matters. Brain-directed approaches appear to work best before neurodegeneration sets in — which is why early diagnosis and newborn screening are so important.