Three Years Clean on Natalizumab, and a Rising Antibody Index
Three years of nothing — no relapse, no lesion, no change on exam — and a laboratory value that has nearly doubled. The number tracks a brain infection, not his disease. Spacing the infusions out lowers it without giving up the drug. Whether that is risk management or postponement is what the group has to decide.
Nothing has changed about D.R.'s multiple sclerosis. He is 34, a structural engineer made site-lead on a bridge-replacement project six months ago, on scaffolding most mornings reviewing rebar placement. Eight months of azathioprine failed to hold two relapses in close succession; natalizumab, started a little over three years ago, has held everything since. No relapses, no new lesions, no change in his exam since the first infusion. The number the group is looking at today measures something else entirely.
His JC virus antibody index has climbed from 1.8 to 3.4 across his last two annual checks. That value does not track disease activity. It tracks the risk of progressive multifocal leukoencephalopathy — an infection natalizumab's own mechanism enables, since blocking lymphocyte trafficking into the central nervous system keeps immune surveillance out of the one compartment where a reactivating virus would otherwise be caught. Bloomgren and colleagues' 2012 stratification built its highest-risk tier from three factors: seropositivity, prior immunosuppressant exposure, and more than two years on the drug. At month 38, with azathioprine behind him, he has all three. Plavina's 2014 work then subdivided that tier by index value, with roughly 1.5 marking the higher band; his has passed it and kept going. Read together, those two papers put him near one in a hundred. What makes this hard is that his exam, his scans and his own account of his life all say the drug is working exactly as intended, and none of them can see the thing the number is watching.
Reviewing the annual labs, three years in
I'd switch him off natalizumab now, to an anti-CD20 agent. Bloomgren's 2012 NEJM stratification puts PML incidence as high as roughly 1 in 100 in patients who are seropositive, immunosuppressant-exposed, and past two years on the drug — he now meets all three. And his index has nearly doubled in eighteen months, which on Plavina's 2014 index data moves him within the seropositive group as well, not just across it. Every additional month on this drug is itself another risk factor accumulating, not a neutral wait.
If his index had stayed flat around 1.8, I wouldn't be pushing this nearly as hard — it's specifically the trajectory, not just crossing the numeric threshold once, that worries me.
I'd stay on natalizumab but move him to extended-interval, six-week dosing rather than stop outright. Zhovtis Ryerson's 2019 TOUCH-registry analysis in Neurology found substantially lower PML incidence on extended dosing in seropositive patients, and NOVA — Foley and colleagues, Lancet Neurology 2022 — randomized that question directly and found no significant increase in disease activity on the six-week schedule against standard four-week dosing.
That's real risk reduction, not risk elimination, and I want to be honest that it doesn't return him to the same baseline risk a JC-negative patient carries. But given how well this specific drug is controlling his disease, I'd rather try to lower the number first than give up something working.
I don't think switching off natalizumab is the risk-free option either of you is implicitly treating it as. Rebound disease activity after natalizumab discontinuation is real and well-documented — relapses and MRI activity that can exceed his pre-treatment baseline, especially if the transition to whatever comes next isn't carefully bridged.
I'm not arguing against ever switching him — I'm arguing that if we do, it has to be planned as a bridged transition to the next agent, not a stop-and-reassess. Whichever path this group picks today, 'and then what covers him in the gap' has to be answered in the same conversation, not left for the next visit.
Agreed: switch to extended-interval, six-week natalizumab dosing, with the JC virus antibody index re-checked every three months rather than annually so the group is watching a trend, not waiting for the next scheduled data point. A bridging plan to ocrelizumab was drafted in advance and filed in his chart, not left to be improvised if the index keeps climbing.
Not agreed: how many more months of a rising index, even on extended dosing, would tip the decision to switch off natalizumab entirely regardless of how well his MS remains controlled. The Neuroimmunologist's position was not overruled, only deferred — today's plan buys time to see whether extended dosing bends the trend, on the explicit understanding that if it doesn't, the group returns to this question, not away from it.