Sponsors developing therapies for diseases that span adult and pediatric populations often have to decide how to define the population in each FDA designation request. Orphan drug designation and rare pediatric disease designation both rely on a prevalence threshold, but the population in each one can be scoped differently. That choice affects whether a priority review voucher is granted, how FDA reads the approved indication against the designated disease, and whether a narrowed subset holds up as a medically plausible population.
The analysis below looks at FDA review records where the orphan designation covered a broad disease and the rare pediatric disease designation was limited to a subtype defined by histology, molecular alteration, or phenotype. For each precedent it sets out how the populations were worded, how FDA judged the subtype boundary, and what happened with the voucher. It also covers cases where FDA rejected a narrowed subset or denied a voucher because the approved indication went beyond the designated pediatric disease.
Broad orphan drug designation, narrow rare pediatric disease designation: FDA precedents and how the populations were defined
Orphan drug designation (ODD) and rare pediatric disease designation (RPDD) share a statutory prevalence test, but they do not have to describe the same population. FDA review records in Drugs@FDA show several products where the orphan designation covered a broad disease (soft tissue sarcoma, NTRK fusion solid tumors, malignant glioma, glioblastoma, Duchenne plus Becker muscular dystrophy) while the RPDD was limited to a pediatric-predominant subtype defined by histology, molecular alteration, or phenotype. The records also include cases where FDA rejected a narrowed subset or refused a voucher because the approved indication reached beyond the designated pediatric disease.
Summary of precedents
| Product (application) | Orphan designation (as recorded) | Rare pediatric disease designation (as recorded) | How the RPDD population was narrowed | Voucher outcome |
|---|---|---|---|---|
| Vitrakvi, larotrectinib (NDA 210861) | "Treatment of soft tissue sarcoma" (Aug 31, 2015) and "treatment of solid tumors with NTRK-fusion proteins" (May 9, 2017) 8640 | "Treatment of infantile sarcoma" (June 16, 2016), called infantile fibrosarcoma elsewhere in the file 86123136 | Histologic subtype. FDA stated that IFS "is a subset of NTRK fusion-driven tumors" 87 | Denied: NDA was for NTRK fusion solid tumors, not IFS 123 |
| Duvyzat, givinostat (NDA 217865) | Treatment of Duchenne muscular dystrophy and Becker muscular dystrophy (Apr 12, 2013) 120 | Treatment of DMD (Sept 22, 2020) 115 | Disease phenotype: DMD only, Becker excluded | Granted, PRV NDA 217865 164 |
| Ojemda, tovorafenib (NDA 217700) | Treatment of malignant glioma (Sept 24, 2020) 208209 | Treatment of low-grade gliomas harboring an activating RAF alteration disproportionately affecting children (July 26, 2021) 209 | Tumor grade, molecular alteration, and pediatric epidemiology written into the condition | Granted, PRV NDA 217700; second voucher for the oral suspension (NDA 218033) denied under the one-voucher-per-drug rule 234235232 |
| Modeyso, dordaviprone (NDA 219876) | Treatment of glioblastoma; treatment of malignant glioma 212 | Treatment of H3 K27M-mutant glioma 212 | Molecular alteration (H3 K27M) | Granted, PRV NDA 219876 213 |
The regulatory framework that allows the split
Two tests, only one of which is age-based
FDA's July 30, 2019 draft guidance Rare Pediatric Disease Priority Review Vouchers requires a rare pediatric disease to meet both of the following criteria. It must be a serious or life-threatening disease whose serious or life-threatening manifestations primarily affect individuals from birth through age 18. It must also be a "rare disease or condition" under section 526 of the FD&C Act 187. The prevalence test is the orphan test: fewer than 200,000 persons in the United States, or the cost-recovery alternative 2.
FDA reads "from birth to 18 years" as ages 0 through 18. The question is whether the disease's manifestations, not simply its onset, are serious or life-threatening in children under the current standard of care. Relevant factors include the timing of progression, effects on growth and development, and whether the proportion of affected children exceeds that of adults 188.
Orphan designation has no age component. A disease that is rare overall can hold an orphan designation even though it is mainly an adult disease. Sponsors therefore often find that a broad orphan-designated disease fails the "primarily affects children" test, while a narrower entity within it passes.
Subsets and subpopulations
The guidance allows a subset of a non-rare disease to qualify, but only as a valid orphan subset. The sponsor must show that use outside the subset would be inappropriate because of a property of the drug, such as toxicity, mechanism of action, or prior clinical experience. The subset must itself be serious or life-threatening, with its serious manifestations primarily affecting persons aged 0 through 18 2. A sponsor seeking RPDD for a subset of a disease affecting 200,000 or more persons must explain why the remaining patients are not appropriate candidates for the drug 3. FDA may deny RPDD when the drug is neither for a rare disease nor for an orphan subset of a non-rare disease 6.
The separate final guidance Clarification of Orphan Designation of Drugs and Biologics for Pediatric Subpopulations of Common Diseases (December 19, 2017) states that FDA no longer expects to grant "pediatric-subpopulation designations" based solely on the pediatric share of a common disease falling below 200,000 7. Earlier designations of that type, such as pediatric ulcerative colitis and pediatric HIV, remain in place 5. Designation remains available for three situations: a rare disease that includes a rare pediatric subpopulation, a pediatric subpopulation that is a valid orphan subset, and a pediatric disease that is actually different from the adult disease 4. FDA judges whether a disease is distinct by looking at pathogenesis, disease course, prognosis, and treatment resistance, considered cumulatively in the context of the particular drug 1.
In the precedents above, the RPDD populations are framed as distinct disease entities (a histologic tumor type, a specific dystrophinopathy, a molecularly defined glioma) rather than age-sliced subpopulations of the broader orphan disease.
Voucher eligibility follows the application, not the designation
The draft guidance says a qualifying rare pediatric disease product application must be for prevention or treatment of a rare pediatric disease and must meet all other statutory criteria 189. A sponsor may seek approval for both pediatric and adult patients with the same rare pediatric disease without losing eligibility, as long as the approved use includes the required pediatric use. An application for adults only is ineligible 190. The guidance does not directly address an application whose indication is broader than the designated subtype. The larotrectinib decision below shows how FDA handled that situation.
Case studies
Larotrectinib: tissue-agnostic orphan designation, infantile fibrosarcoma RPDD
Larotrectinib's orphan history started with "treatment of soft tissue sarcoma" (August 31, 2015). A second designation, "treatment of solid tumors with NTRK-fusion proteins," followed on May 9, 2017. The RPDD, granted June 16, 2016, is recorded in the regulatory-activity table as "treatment of infantile sarcoma" 8640. The approval-letter voucher decision and the NDA review summaries identify the designated disease as infantile fibrosarcoma (IFS) 123136.
FDA described the development program as tissue-agnostic, covering adult and pediatric patients with NTRK fusion tumors 87. The proposed indication was adult and pediatric patients with locally advanced or metastatic solid tumors harboring an NTRK gene fusion 136. FDA stated explicitly that IFS "is a subset of NTRK fusion-driven tumors" 87. The review notes that NTRK fusions occur in more than 90% of certain rare cancers, including infantile fibrosarcoma 79. OOPD advised Loxo to consider amending the orphan designation from soft tissue sarcoma to the NTRK fusion indication. FDA also noted that whether a voucher could be supported depended on how the disease supported by the trial data would ultimately be defined 163.
Loxo argued that the tissue-agnostic indication and IFS were not different indications because they share the same underlying pathophysiology 40. FDA denied the voucher. It found that the NDA was not an application for a rare pediatric disease: the RPDD covered IFS, but the application sought approval for NTRK fusion solid tumors. FDA determined that the serious and life-threatening manifestations of NTRK fusion solid tumors do not primarily affect individuals from birth through age 18 123.
The practical point is that the IFS designation was valid, but it did not carry over to a broader, mixed-age molecular indication that included it.
Givinostat: DMD and Becker orphan designation, DMD-only RPDD
Givinostat received orphan designation on April 12, 2013 for the treatment of Duchenne muscular dystrophy and Becker muscular dystrophy 120. The RPDD, granted September 22, 2020, covered DMD only 115. FDA's published review documents do not explain why Becker was left out of the RPDD. At approval, FDA granted Italfarmaco a rare pediatric disease priority review voucher (PRV NDA 217865) 164. Here the approved indication stayed inside the narrower designated disease, so the broader orphan designation did not stand in the way of voucher eligibility.
Tovorafenib: malignant glioma orphan designation, RAF-altered low-grade glioma RPDD
Tovorafenib received orphan designation on September 24, 2020 for the treatment of malignant glioma 208209. On July 26, 2021, FDA granted RPDD for "treatment of low-grade gliomas harboring an activating RAF alteration disproportionately affecting children" 209. The RPDD wording includes three limits: tumor type (low-grade glioma), molecular alteration (activating RAF alteration), and a statement about pediatric epidemiology 209. The records do not settle whether FDA considered this a strict subset of "malignant glioma," so the relationship is better described as a narrower, differently framed condition than as a formal nested subset.
The approved population was narrower again. It covered pediatric patients 6 months and older with relapsed or refractory pediatric low-grade glioma harboring a BRAF fusion or rearrangement, or a BRAF V600 mutation 80. FDA granted a voucher for the tablets (PRV NDA 217700) 234235. It denied a second voucher for the oral suspension (NDA 218033) because section 529(g) bars a sponsor from receiving more than one voucher for the same drug 232.
Dordaviprone: glioblastoma and malignant glioma orphan designations, H3 K27M-mutant glioma RPDD
Dordaviprone (ONC201) holds orphan designations for the treatment of glioblastoma and for the treatment of malignant glioma. Its RPDD is for the treatment of H3 K27M-mutant glioma 212. The RPDD is narrower by molecular alteration and has no age qualifier 212. The labeled pediatric population covers patients aged 1 year and older with diffuse midline glioma harboring an H3 K27M mutation and progressive disease after prior therapy 205. FDA granted a voucher (PRV NDA 219876) 213. FDA's published review documents confirm the grant but do not describe FDA's specific eligibility reasoning 213.
Where FDA would not accept a narrower pediatric population
Defibrotide: post-HSCT subset of hepatic VOD
Defibrotide received orphan designation for treatment of hepatic veno-occlusive disease (VOD) on May 21, 2003. It also holds designations for prevention of hepatic VOD 175168. For the voucher, the sponsor framed the disease as hepatic VOD with multi-organ dysfunction following hematopoietic stem-cell transplantation (HSCT) 17022.
OOPD agreed that hepatic VOD was an orphan disease. It found, however, that the sponsor had not justified limiting the pediatric analysis to post-HSCT VOD. Unless product characteristics ruled out use in other causes of VOD, the population estimate had to include all hepatic VOD, with evidence that more than 50% of incident cases occur in persons aged 0 to 18 119170. Even within the post-HSCT population, OOPD's 2013 calculation estimated 602 pediatric cases against 2,031 adult cases 172. The sponsor later broadened its framing to all hepatic VOD 167. FDA denied the voucher because the sponsor did not show that the indication was for a rare pediatric disease that primarily affects individuals from birth to 18 years 35.
Defibrotide shows the limit on subset framing. A clinically defined setting within a broad orphan disease does not qualify for RPDD unless it is a valid orphan subset (supported by properties of the drug) and it still primarily affects children.
Tezacaftor/ivacaftor: an age- and genotype-limited request for cystic fibrosis
Tezacaftor/ivacaftor received orphan designation for the treatment of cystic fibrosis on June 15, 2017 20. Vertex requested RPDD for a narrower population: patients with CF aged 12 years and older who are homozygous for F508del or who have at least one CFTR mutation 20. OOPD rejected the sponsor's first argument, which relied mainly on more than half of CF patients being 18 or younger. OOPD noted that onset in childhood is not enough and that moderate to severe lung disease more often affects adults 180. OOPD still found serious manifestations that primarily affect children: progression to end-stage liver disease, pancreatic insufficiency causing failure to thrive and growth retardation, and complications of meconium ileus, intussusception, and fibrosing colonopathy 18067. The reviewer recommended RPDD for "tezacaftor/ivacaftor for treatment of CF," which is the whole disease rather than the requested age- and genotype-limited population 67.
FDA then denied the voucher for a different reason. The NDA did not rely on clinical data from studies in a pediatric population using pediatric-intended doses, as section 529(a)(4)(D) requires. It relied instead on adult studies that included some subjects aged 12 and older 33. The review also noted that the pivotal endpoint, pulmonary function, measured a manifestation not considered primarily pediatric 69.
Practical implications for designation strategy
- Define the RPDD disease as a recognized entity. In the successful precedents, the RPDD condition was a disease in its own right, such as infantile fibrosarcoma, DMD, RAF-altered low-grade glioma, or H3 K27M-mutant glioma, rather than the pediatric share of a broader disease 86115209212. That fits the guidance's "different disease" and "valid orphan subset" routes 14.
- Justify any subset with properties of the drug. A clinical setting chosen for convenience, such as post-HSCT VOD, will be expanded back to the whole disease unless the drug's properties justify the boundary 11923.
- Expect the orphan designation to be broader. Holding a broad orphan designation alongside a narrow RPDD is not a problem in itself. Givinostat, tovorafenib, and dordaviprone all received vouchers with this arrangement 164234213.
- Match the marketing application to the RPDD. The larotrectinib denial shows that an indication broader than the designated pediatric disease, especially a mixed-age molecular indication, can lose voucher eligibility even when a valid RPDD exists 123.
- Build pediatric data into the pivotal package. The tezacaftor/ivacaftor denial shows that a valid RPDD does not rescue an application built mainly on adult studies 33.
- Plan one voucher per drug. Additional dosage forms of the same drug do not earn additional vouchers 232.
Questions that FDA's published record leaves open, and that are worth asking FDA directly, include FDA's reasoning for excluding Becker from the givinostat RPDD, the exact orphan designation wording for other pediatric-predominant products such as mirdametinib, and how FDA handles supplements that later expand an indication beyond the designated pediatric subtype.