In This Issue

Can you regrow your knee cartilage from skin cells?

🧬  The pitch: FibroBiologics just landed an EU patent to hand your ordinary skin cells a single gene and coax them into becoming cartilage.

⚠️  The catch: The most advanced attempt at this just failed. Is this pseudo-science?

💰  Follow the money: The current winning companies include Biosplice, who rode a US$12B valuation down to a decade-long grind, and FibroBiologics chases a tens-of-billions market on single-digit-millions of cash.

🇪🇺  The EU filing: This is a European patent, in the one region that keeps approving cell therapies and then losing them off the market.

Can you regrow your knee?

You are 58 and you can no longer ignore that deep grinding ache that means the cushion between your bones has worn thinner than it was last year. Your doctor already told you the cartilage is going, and cartilage, everyone agrees, does not come back.

Cartilage is one of the few tissues in your body with almost no ability to repair itself. It lacks blood supply and has no nerves, and has very few of the spare cells that heal a cut on your arm within a week. Once it wears down, the standard menu runs from painkillers to steroid injections to, eventually, a surgeon removing the joint and bolting in a metal-and-plastic one.

What if a clinician can take a few skin cells, the ordinary fibroblasts that sit under every square inch of you, hands them a single gene, and injects them into the joint. The cells read the instruction and start behaving like cartilage cells. The joint, in theory, begins to rebuild the surface it lost.

That is the future a recently published EU patent points toward.

The company is FibroBiologics, a clinical-stage biotech based in Houston and listed on the Nasdaq as FBLG. Their whole platform rests on the idea that fibroblasts, the workhorse cells of connective tissue, are cheap to get and more versatile than they look.

The patent describes taking those fibroblasts and delivering a gene called Sox9 into them. Sox9 is a master switch for cartilage. When it is active, it tells a cell to start producing the proteins that make up cartilage and give the tissue its cushioning.

The delivery vehicle is an expression vector, a loop or strand of DNA that carries the gene into the cell and gets it switched on. The patent covers both viral versions (which smuggle the gene in efficiently) and plasmid versions (a non-viral loop of DNA, simpler and safer to handle).

Once the gene is active, the fibroblast is meant to change identity and become a chondrocyte, or something close enough to pass for one. In the patent's language, the treated cell starts to secrete the markers a real cartilage cell would.

There are two routes. First, convert the cells outside the body and then inject the finished product into the joint. Second, deliver the gene directly into the joint and let fibroblasts already living there, or nearby, do the converting on site.

The patent is deliberately written around the joint being a hostile place, which is avascular, aneural, low on oxygen. One claim specifically covers using a promoter (the on-switch that controls how hard a gene runs) tuned to work in exactly that low-oxygen environment. Another adds a reservoir of oxygen and nutrients delivered alongside the cells.

Cartilage does not heal. The tissue is avascular, so it never gets the flood of repair cells and nutrients a bleeding wound does. Damage it, and the body mostly leaves the gap.

The current fixes each carry a catch. Microfracture drills into bone to provoke a clot, but the tissue that grows back is weaker fibrocartilage. Autologous chondrocyte implantation harvests healthy cartilage from the patient, which means cutting good tissue to patch bad and limits how much material you have. Total joint replacement works, until the implant loosens or fails.

Every existing option either transplants scarce tissue, grows an inferior substitute, or replaces the joint with hardware. Nothing currently exists that can make the body regrow its own cartilage from a cell it can afford to lose.

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Cartilage regeneration splits into two camps: the companies growing or implanting cartilage cells, and the smaller group trying to reprogram other cells into cartilage. FibroBiologics sits in the second, thinner camp, and its choice of starting cell is the differentiator.

The closest cautionary cousin is Kolon TissueGene. Its product, TG-C (sold in South Korea as Invossa), is an allogeneic cell-and-gene therapy. The cartilage cells are engineered to pump out a growth factor, injected into the knee. It became the world's first approved gene therapy for osteoarthritis in Korea in 2017. Then regulators discovered one of its two cell components was mislabeled, actually derived from kidney cells, and pulled the approval in 2019. Its U.S. Phase 3 program limped on for years and, in July 2026, the first pivotal trial missed both its primary endpoints.

Vericel is the incumbent that actually sells cartilage repair. Its MACI product takes a patient's own chondrocytes, grows them on a collagen membrane, and implants them. It works, it is FDA-approved, and it addresses a market Vericel pegs at around US$3 billion. But it still relies on harvesting the patient's cartilage and an open surgical step, the exact friction FibroBiologics is trying to skip.

Then there are the adjacent bets on osteoarthritis that avoid gene therapy entirely. MEDIPOST's Cartistem uses umbilical-cord stem cells and reported positive Phase 3 results in Japan in 2026. Biosplice filed for approval of lorecivivint, a small-molecule injection, in early 2026. Organogenesis has an FDA-accepted application for an amniotic-tissue product. Each is looking at a disease-modifying treatment for a joint that currently only gets symptom relief, from a different scientific direction.

FibroBiologics is broader and earlier than any of them here. It holds 270-plus issued and pending patents across wound healing, disc degeneration, psoriasis and more, and cartilage is one filing in that stack. The uncomfortable question is whether the fibroblast-to-chondrocyte conversion is genuinely more robust than the cartilage-cell approaches that have already stumbled.

Osteoarthritis is the demand behind all of it, and the demand is enormous. The Arthritis Foundation projects at least 130 million people worldwide will be affected by 2050.

Estimates for the cartilage repair market cluster loosely between US$1.8 billion and US$6 billion for 2024-25. Grand View Research put 2024 at US$5.98 billion growing at 5.1%, while Persistence Market Research sized 2025 at US$1.85 billion growing at 7.9%.

The broader classification of regenerative-medicine-for-cartilage specifically was valued around US$4.68 billion for 2025 by Research and Markets, and cell-based approaches already hold the majority share. Widen it again to the disease driving all of it, and you reach the degenerative disc disease market alone, which FibroBiologics targets with its lead candidate, valued at roughly US$34 billion in 2024 with projections toward US$66 billion by 2033, per figures compiled by Trefis.

Cartilage failure sits at the top of a funnel. A worn knee that could have been repaired early becomes an osteoarthritic knee, then a replaced knee, then sometimes a revision surgery. Every dollar of cheap early intervention that actually worked would redirect spending away from the expensive hardware end of that funnel.

The money in cartilage is concentrating around companies that already have products in patients, and moving away from the ones still proving the science. This quarter sharpened that pattern.

FibroBiologics itself shows what small looks like in this field. The company went public on the Nasdaq via direct listing, entered 2025 with about US$14 million in cash against a 2024 net loss of US$11.2 million, and signed a Standby Equity Purchase Agreement with Yorkville Advisors for financial flexibility. Its most advanced program is actually CYWC628 for diabetic foot ulcers, which dosed its first patients in a Phase 1/2 trial in Australia in mid-2026. Its disc-degeneration candidate, CybroCell, has held an FDA IND clearance since 2018 but has not yet begun dosing. The cartilage work described in this patent sits earlier still.

Vericel is the number that anchors the whole category. The company generated record 2025 revenue of US$276.3 million, with MACI alone bringing in US$239.5 million, up 21% year over year and the third straight year of 20%-plus MACI growth. It posted net income of US$16.5 million and ended the year with roughly US$200 million in cash and no debt. It is a proven autologous implant sold to surgeons, compounding at 20% a year. Vericel is now taking MACI toward a UK launch planned for 2027, the reverse of the usual direction of travel for cell therapy and a reminder that the money in cartilage today sits with the least experimental approach on the board.

The core risk here is that the science underneath this specific claim is genuinely unproven, and the one company that got closest to proving it just failed.

Look again at Kolon TissueGene. Its therapy was built on the same broad premise, put engineered cells into a joint and coax cartilage repair, and it even had an actual product and years of patient data. In July 2026 its pivotal U.S. Phase 3 trial missed both primary endpoints, showing no statistically significant improvement in pain or function over placebo after twelve months. The company blamed a strong placebo response.

Sox9 flipping a skin cell toward cartilage in a dish is well-supported cell biology. Sox9 flipping enough cells, for long enough, in a starved and load-bearing human joint, to rebuild a surface that measurably reduces pain, is a claim with no clinical validation behind it at all.

The patent leans on the joint being avascular and low-oxygen, and claims workarounds (oxygen reservoirs, hypoxia-tuned promoters). But a therapy that only works if you also solve oxygen delivery, cell survival, and durable gene expression inside the joint is really several hard problems stacked into one injection.

A converted fibroblast is a cell whose identity has been rewritten and injected into a joint you cannot easily flush out. If those cells overproduce matrix, form the wrong tissue, or behave unpredictably years later, what happens then and what does the long-term monitoring even look like?