The Disease Burden
Rare diseases affect an estimated 25 to 30 million people in the United States and roughly 300 million globally, spread across more than 7,000 distinct conditions, the majority of them genetic and diagnosed in childhood. Since the Orphan Drug Act of 1983, FDA has granted more than 6,500 orphan drug designations, and orphan indications now account for close to half of all novel drug approvals each year, a share that has climbed steadily as biotech shifted R&D dollars toward small, genetically defined patient populations where clinical trials are smaller, pricing power is higher and regulatory pathways like accelerated approval and priority review vouchers are more accessible.
The economics are unusual by design. A drug that serves a few hundred or a few thousand patients cannot recoup development costs through volume, so pricing runs into the hundreds of thousands or millions of dollars annually, and payers increasingly demand outcomes-based contracts and multi-year installment models, exemplified by CMS's Cell and Gene Therapy Access Model, which launched state-level outcomes-based agreements for sickle cell gene therapies in 2025.
Main Drug Classes and Mechanisms
The rare disease armamentarium spans five broad mechanistic categories. Enzyme replacement therapy, pioneered by Genzyme (now Sanofi) with Cerezyme for Gaucher disease and Fabrazyme for Fabry disease, remains a backbone for lysosomal storage disorders, alongside BioMarin's Naglazyme and Aldurazyme. Antisense oligonucleotides and RNAi therapeutics have matured rapidly: Biogen and Ionis's Spinraza (nusinersen) for spinal muscular atrophy, Alnylam's Onpattro and Amvuttra (patisiran and vutrisiran) and AstraZeneca/Ionis's Wainua (eplontersen) for hereditary transthyretin amyloidosis all silence or modulate disease-causing mRNA.
Small-molecule splicing modulators and correctors represent a third pillar, best illustrated by Vertex's Trikafta (elexacaftor/tezacaftor/ivacaftor) for cystic fibrosis and Roche/PTC Therapeutics' Evrysdi (risdiplam) for SMA, both of which correct protein production rather than replacing missing enzymes. Monoclonal antibodies targeting complement, exemplified by Alexion/AstraZeneca's Soliris and Ultomiris (eculizumab and ravulizumab) for paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome, form a fourth category. The newest and most consequential class is one-time genetic medicine: AAV-based gene therapy and, since late 2023, CRISPR-based gene editing.
Leading Marketed Products and Manufacturers
Gene therapy has moved from experimental to commercial reality across several indications. Novartis's Zolgensma (onasemnogene abeparvovec) for SMA, priced near $2.1 million, competes with Spinraza and Evrysdi in a three-way franchise fight. In hemophilia, CSL Behring's Hemgenix (etranacogene dezaparvovec) for hemophilia B carries a list price around $3.5 million, while BioMarin's Roctavian (valoctocogene roxaparvovec) for hemophilia A and Pfizer's Beqvez, approved in 2024, round out a field still working through durability and payer-adoption questions.
Sickle cell disease saw the first FDA-approved CRISPR therapy in December 2023: Casgevy (exagamglogene autotemcel), developed by Vertex Pharmaceuticals and CRISPR Therapeutics, priced at roughly $2.2 million, alongside bluebird bio's lentiviral gene therapy Lyfgenia (lovotibeglogene autotemcel) at about $3.1 million. In transthyretin amyloidosis, Pfizer's Vyndaqel and Vyndamax (tafamidis) generate billions in annual revenue and compete against Alnylam's RNAi franchise and AstraZeneca's Wainua. PTC Therapeutics continues to anchor Duchenne muscular dystrophy alongside Sarepta's Elevidys, the first gene therapy approved for DMD, now facing safety scrutiny after reported patient deaths in 2025 that triggered a partial label restriction.
Patent Cliffs and Biosimilar Dynamics
Orphan exclusivity, typically seven years under the Orphan Drug Act plus additional biologics exclusivity, is now expiring across the first generation of blockbuster rare disease biologics, and biosimilar competition has finally arrived. Amgen's Bkemv (eculizumab-aeeb) won FDA approval in May 2024 as the first biosimilar to Soliris, followed by Samsung Bioepis's Epysqli, putting real pricing pressure on Alexion's roughly $2,000-per-vial complement franchise for the first time. Pfizer and Protalix's Elelyso has chipped at Sanofi Genzyme's Cerezyme in the Gaucher market for years, and additional biosimilar filings targeting Fabrazyme and Naglazyme are working through FDA's pathway.
This erosion is reshaping corporate strategy. Companies that built franchises on recombinant enzymes and early monoclonal antibodies are pivoting R&D and business-development spend toward next-generation modalities that are harder to biosimilar, namely gene therapies and RNA-based drugs, where manufacturing complexity and clinical durability data create higher barriers to follow-on entry. AstraZeneca's 2021 acquisition of Alexion for $39 billion and Vertex's continued reinvestment of CF cash flows into gene editing and pain programs both reflect this defensive repositioning against looming biosimilar and small-molecule generic threats, including generic ivacaftor combinations now advancing through litigation with Indian manufacturers.
Where the Pipeline Is Heading
In vivo gene editing is the clearest frontier. Intellia Therapeutics and Regeneron's NTLA-2001, a CRISPR-based in vivo knockdown for ATTR amyloidosis delivered via lipid nanoparticle rather than AAV, is advancing through late-stage trials and could become the first systemically administered CRISPR therapy that avoids ex vivo cell manipulation altogether. Beam Therapeutics is pursuing base editing for sickle cell disease with BEAM-101, aiming to avoid the double-strand breaks associated with first-generation CRISPR. Wave Life Sciences and Korro Bio are advancing RNA editing platforms designed to correct point mutations without permanently altering DNA, a potentially safer alternative for chronic dosing.
Protein replacement is also going mRNA: Moderna has multiple rare metabolic disease programs, including mRNA-3927 for propionic acidemia, in mid-stage development, betting that transient protein expression can substitute for enzyme infusions or gene therapy in select disorders. Meanwhile, AAV capsid engineering, led by companies including Regenxbio and Ultragenyx, is focused on reducing immunogenicity and enabling redosing, a major limitation of current one-shot gene therapies that cannot currently be readministered once neutralizing antibodies develop.
The next five years will test whether payers, health systems and manufacturers can sustain a business model built on seven-figure one-time cures layered on top of a shrinking pool of high-margin legacy biologics now facing biosimilar entry. Expect continued consolidation as large-cap pharma acquires clinical-stage gene and RNA editing platforms to backfill pipelines, more outcomes-based reimbursement experiments modeled on the CMS sickle cell framework, and growing regulatory attention to long-term safety signals from AAV therapies following the Elevidys episode, all of which will determine whether genetic medicine becomes the durable second act of the orphan drug era or a costly cautionary tale.