What it is
Romiplostim is a peptide built into an antibody frame. The Nplate label describes it as "an Fc-peptide fusion protein (peptibody)": two identical chains, each made of the Fc fragment of human immunoglobulin G1 — the tail end of an antibody — with a peptide carrying two receptor-binding domains attached at its far end. The two chains pair through the Fc portion, so one molecule carries four copies of the binding domain. It is made in E. coli by recombinant DNA technology.
The peptide is the working part and the antibody fragment is the delivery system. A short peptide on its own is filtered and broken down quickly; joined to an Fc fragment, it is recycled through the same route that keeps antibodies in the blood for days. That is what allows a once-weekly injection. The whole assembly is far past the 40-residue line where a peptide becomes a protein — the "peptide" in peptibody names its active part, not its size.
How it works
The peptide binds and activates the thrombopoietin (TPO) receptor on megakaryocytes, the bone-marrow cells that shed platelets — "a mechanism analogous to endogenous TPO" (label). The unusual point is that romiplostim "has no amino acid sequence homology to endogenous TPO." Its binding peptides were engineered rather than copied from the hormone — the molecule switches on the same receptor with a different key.
What the label's trials show
| Trial (label) | Who | Result |
|---|---|---|
| Study 1, NCT00102336 | 62 adults with ITP, spleen intact, 24 weeks, 2:1 vs placebo | Durable platelet response 61% vs 5%; overall response 88% vs 14%; rescue therapy 20% vs 62% |
| Study 2, NCT00102323 | 63 adults with ITP after spleen removal, 24 weeks | Durable response 38% vs 0%; overall 79% vs 0%; rescue therapy 26% vs 57% |
| Study 3, NCT01143038 | 75 adults within ~2 months of diagnosis, open-label, 12 months, no control arm | 93% reached ≥50 × 10⁹/L; 32% kept every count ≥50 for 6+ months off all ITP drugs |
A "durable" response meant a weekly count of at least 50 × 10⁹/L in 6 of the last 8 weeks without rescue medication. Across the two controlled trials, bleeding of grade 2 or worse was reported in 15% vs 34%, and serious bleeding in five romiplostim and four placebo patients. Study 3 had no comparison group, so its remission figure cannot separate the drug's effect from ITP's own tendency to settle early in its course.
The second approved use — survival after radiation that destroys the bone marrow — was added in 2021. Human trials of that exposure cannot be run ethically, so the label's evidence for it comes from animal studies; the evidence column above counts it as animal-based.
Safety
The label's leading warning concerns myelodysplastic syndromes (MDS): a placebo-controlled trial of 250 adults with low-risk MDS was terminated because more cases of acute myeloid leukaemia appeared on romiplostim (10 of 167, 6.0%, vs 4 of 83, 4.8%, in the 58-week phase); at five years the rates were 11.9% vs 11.0%. Romiplostim is not indicated for MDS. Other warnings: blood clots as platelets rise, including portal vein thrombosis in people with chronic liver disease, and loss of response, sometimes from neutralising antibodies. In adults, the most common adverse reactions at least 5% more frequent than on placebo were joint pain, dizziness, insomnia, muscle pain, limb pain, abdominal pain, shoulder pain, indigestion and tingling (label). Dosing is by body weight and is set and adjusted by the treating team.
Where it stands
- Approved in the US since 2008-08-22 (Nplate, BLA 125268, Amgen), in 125, 250 and 500 mcg vials; children with ITP added by a supplement approved 2018-12-14; radiation injury added 2021-01-28 (Drugs@FDA, 2026-10-15).
- One application: openFDA's Drugs@FDA data lists a single romiplostim application, Amgen's BLA 125268 (2026-10-15).
- Research record: PubMed indexes 1,035 records, 38 tagged as randomised trials; ClinicalTrials.gov lists 123 studies (2026-10-15).
Related reading
For where a peptide ends and a protein begins, see peptide vs protein. Another approved drug that acts on platelets — by blocking them rather than making more — is the cyclic peptide eptifibatide. Another peptide drug made by recombinant DNA technology is teriparatide, the bone-building PTH fragment.