Patients who rely on a Sanofi drug for Pompe disease — a rare, progressive muscle-wasting disorder — are being unsettled by reports of delays in getting their medicine.

Nexviazyme (also sold as Nexviadyme) is an enzyme replacement therapy supplying an enzyme people with Pompe disease cannot make. Patient advocates have reported shipment delays for some deliveries and urged patients to confirm upcoming shipments; whether Sanofi’s older Pompe drug Lumizyme is affected has not been confirmed.

What Pompe disease does

The disease is caused by deficiency of an enzyme that breaks down glycogen inside cells. Without it, glycogen accumulates progressively in lysosomes and damages muscle tissue.

Muscle is affected throughout the body, and the diaphragm and respiratory muscles are particularly vulnerable. Respiratory failure is the usual cause of death in the disease, which is why interruptions in treatment matter more than the phrase “muscle-wasting disorder” conveys.

The infantile-onset form progresses rapidly and was uniformly fatal in early childhood before enzyme replacement existed. Later-onset forms progress more slowly over years or decades but follow the same course.

Why missed doses matter here specifically

Enzyme replacement is given by intravenous infusion, typically every two weeks, and the reason the schedule is tight is pharmacological.

The infused enzyme is cleared from the body within days. It does not accumulate or persist — it is taken up by cells, does its work and is degraded. Enzyme activity therefore falls between infusions by design, and the interval is set to keep glycogen accumulation suppressed on average.

Extending that interval means a period with no enzyme activity, during which glycogen accumulates unchecked. Damage from that accumulation is not fully reversible, so a delay does not simply postpone benefit — it can cost function permanently.

The manufacturing backdrop

The delays follow an FDA warning letter issued in mid-2026 after a January inspection of a Sanofi facility in Waterford, Ireland.

A warning letter is a formal notice of significant regulatory violations. It does not itself halt production, and it obliges the company to respond with corrective actions and creates pressure to tighten quality procedures — which can slow batch release even when nothing is wrong with the product.

Based on available information, the situation appears to be a product-release and distribution-timing issue rather than a safety or quality problem with medicine already released. The scope and duration remain uncertain.

Why biologics are fragile to supply shocks

An enzyme replacement therapy cannot be sourced elsewhere at short notice, and the reasons are structural.

It is produced in engineered cells in bioreactors, and each production run takes weeks. The manufacturing process is registered with regulators as part of the approval, so producing the same drug at a different site requires regulatory clearance that takes months at minimum.

There is no generic equivalent to substitute, and typically a single global facility supplies the world. That combination means a disruption at one site propagates to every patient with no available workaround.

Why it matters

For rare-disease patients, supply interruptions carry outsized stakes: there are few or no alternatives, and consistent dosing is essential.

Nexviazyme is a growing product for Sanofi — €426 million in the first half of 2026, up about 14.5% — which makes reliable supply commercially as well as clinically important. Affected patients should contact their care team and Sanofi’s patient-support channels about their specific shipments.

What patients can practically do

Individual patients have limited options during a supply interruption, but they are not none.

Confirming shipment dates in advance rather than assuming delivery is the immediate step advocates recommend, because knowing early creates time to arrange alternatives. Care teams may be able to source doses through hospital pharmacy stock, other infusion centres, or the manufacturer’s patient-support programme, which typically maintains emergency supply arrangements.

Where an interval must be extended, clinicians can prioritise. Patients with the most advanced respiratory involvement, and infants with rapidly progressing disease, have the least tolerance for a missed dose — and triaging limited supply toward them is preferable to distributing shortfall evenly.

What patients should not do is adjust their own regimen, spacing infusions further apart to conserve supply without clinical guidance. The dosing interval is set for a reason, and self-managed extension risks precisely the irreversible loss the schedule exists to prevent.

The wider single-source problem

The vulnerability visible here applies across rare disease and is structural rather than a Sanofi-specific failure.

Producing a biologic for a few thousand patients worldwide does not support duplicate manufacturing capacity. A second qualified site costs a similar amount to the first, must be maintained and inspected, and would produce a product with no market unless the primary site failed.

Regulators have encouraged supply-chain redundancy for critical medicines, and the economics resist it most strongly for exactly the drugs where interruption is least survivable. There is no obvious market solution — which is why supply disruption in ultra-rare disease keeps recurring, and why patient organisations have become the effective early-warning system.

This is a developing story and general information, not medical advice. Patients should not change treatment on their own.