C3 Glomerulopathy After MMF Failure: Iptacopan Now, or One More Traditional Trial
A single patient whose C3 glomerulopathy has already failed one traditional regimen. The disagreement is whether a complement-targeted drug should be the next step, or whether traditional therapy simply wasn't given a full enough trial yet.
Elena V. was diagnosed with C3 glomerulopathy eleven months ago after a routine urinalysis during a pregnancy consult — she wasn't pregnant, it turned out, but the protein in that screening sample sent her down a workup that ended in a kidney biopsy and a diagnosis she'd never heard her own doctors mention before. The pattern was dense deposit disease, the more severe end of the C3G spectrum, with a persistently low C3 and a positive C3 nephritic factor — an autoantibody that keeps the alternative complement pathway's C3 convertase locked in an active state, which is closer to the actual mechanism driving her kidney injury than any generic description of 'immune complex disease' would suggest. She's spent the nine months since on mycophenolate and prednisone, the traditional first-line approach extrapolated mostly from lupus nephritis experience rather than a dedicated C3G trial, and her proteinuria hasn't moved: 3.2 grams a day at diagnosis, 3.2 grams a day now.
That flat trajectory, with a positive C3 nephritic factor sitting right there in her workup, is what makes iptacopan's case concrete rather than theoretical. C3G is fundamentally a complement-pathway disease — the nephritic factor is direct mechanistic evidence her injury is complement-driven, not merely complement-associated — and iptacopan works upstream of that exact process, inhibiting factor B in the alternative pathway rather than broadly suppressing her immune system the way mycophenolate does. APPEAR-C3G tested iptacopan on top of background therapy and found real, significant proteinuria reduction, evidence specific enough to the disease's own mechanism that it reads less like a generic new immunosuppressant and more like a targeted correction. What complicates a simple switch is that C3G's driver varies patient to patient — not everyone has a nephritic factor, some have a pathogenic complement-regulatory gene variant instead — and picking the right inhibitor, or confirming the mechanism at all, arguably matters as much as deciding to use one.
In clinic, at the 9-month reassessment
Nine months, unchanged proteinuria — MMF and steroids haven't worked. C3G is fundamentally a complement-driven disease, and iptacopan targets that pathway directly rather than broadly suppressing her immune system the way we've already tried and failed to do. APPEAR-C3G showed real proteinuria reduction on this exact drug. I'd switch now.
I take the failure seriously, but we've only tried one traditional mechanism — an antimetabolite. We haven't tried a calcineurin inhibitor, which works differently and has decades of use behind it. Iptacopan is newer, more expensive, and its long-term safety picture is still being written. I'd want to try a genuinely different traditional option before reaching for it.
Calling MMF failure 'traditional therapy failure' generally overstates it — we've tried one traditional mechanism, not the traditional approach as a whole.
I'd frame this differently than either of you. It's not really about whether we've exhausted traditional options or whether nine months of failure alone justifies switching — it's that her workup already tells us something concrete: she's C3 nephritic factor positive. That's direct evidence her disease is driven by an alternative-pathway convertase that iptacopan specifically targets.
That's a stronger basis for switching than either 'MMF failed' or 'try a calcineurin inhibitor first' on their own — we're not guessing at a mechanism and hoping a drug matches it, we've confirmed the mechanism is present. I'd switch to iptacopan now, specifically because her own complement workup supports it, not just because the prior regimen didn't work.
Agreed: discontinue mycophenolate, start iptacopan, taper prednisone over 6 weeks. Proteinuria and C3 level rechecked at 3 and 6 months, explicitly justified by her positive C3 nephritic factor rather than treatment-failure reasoning alone.
The second-traditional-regimen position was acknowledged as reasonable in the abstract but was set aside specifically because her own complement workup already identifies the mechanism iptacopan targets — a case-specific argument, not a general preference for newer drugs over established ones.