Krystal's KB113 Shrank 83% of Lesions. The Trial Had Three Patients.

Sixty-nine lesions. Eighty-three assessed. Six injections.

The arithmetic comes from a pilot study in Gorlin syndrome, a rare inherited disorder that leaves patients with dozens, sometimes hundreds, of basal cell carcinomas over a lifetime. The standard response is surgery, again and again, with scar tissue accumulating wherever tumors are removed. Three adults in the study completed a 14-week course of KB113, an experimental therapy built on a modified herpes simplex virus type 1. At the final assessment, 30 days after the last dose, 69 of 83 tracked lesions had shrunk to some degree.

That is why Krystal Biotech, the Pittsburgh company behind the therapy, has opened discussions with the FDA about a larger trial. It is also why the result deserves a slow read. Three patients can show that a drug does something. They cannot show what it will do across a population.

What the lesion counts show

Investigators tracked 83 “landmark” lesions across the three patients. Thirty-six were injected with KB113. Forty-seven were not. To qualify for tracking, a lesion had to measure at least 5 mm on the face or 9 mm elsewhere, large enough to judge by eye. Each patient received six intralesional doses over 14 weeks, with the first two spaced two weeks apart and later doses given every three weeks.

Response was scored on a five-point investigator scale, from worse to complete clearance. There was no placebo arm, no blinded central review and no long-term follow-up. The efficacy data are an unaudited snapshot dated 17 September 2026, disclosed in an 8-K and presented at the PeDRA conference in Alexandria, Virginia, on 15 October. Read the numbers with those limits in view.

Investigator-assessed clinical regression at the final visit, 30 days after the last dose. Source: Krystal Biotech 8-K, October 2026

Half the lesions lost more than half their size. Eight of 83 came within a whisker of disappearing. For each patient, the investigators biopsied one injected lesion that had regressed by at least 90%. All three biopsies showed complete response, with no residual tumor tissue under the microscope.

The non-injected lesions are the part that raised eyebrows. Their regression rate was slightly higher than the injected group, 85.1% against 80.6%. A local injection producing distant shrinkage is the definition of an abscopal effect, and it is the mechanism the company is betting on.

A virus used as a cytokine factory

KB113 is a replication-defective, non-integrating HSV-1 vector. It carries the human genes for interleukin-12 and interleukin-2, two cytokines that stir both innate and adaptive immune responses. The virus is injected into a tumor, where it infects cells and turns them into local producers of those signals. The goal is to make the injected lesion a training ground for T cells that then recognize tumors elsewhere.

That is a different bet from the ex vivo cell-therapy model, which requires extracting a patient’s cells, engineering them outside the body and reinfusing them. KB113 asks whether a redosable virus can do the teaching in place, with no cell collection and no lymphodepletion.

Krystal has been here before, in a different disease. Its commercial product, Vyjuvek, is also built on HSV-1. Krystal describes it as the first redosable gene therapy approved in the United States, Europe and Japan. It treats dystrophic epidermolysis bullosa, a genetic skin disease that leaves patients with fragile, blistering skin. The company’s pitch is that repeated topical dosing of a viral vector is now a solved manufacturing and regulatory problem. KB113 tests whether the same delivery system can carry an immune payload instead of a structural protein.

CHRONICLE: KB113
────────────────────────────
2023  ·  ✅  Vyjuvek approved in the US
2026  ·  🧪  Gorlin pilot enrolls three adults
SEP 17 ·  📊  Efficacy snapshot cut
OCT 15 ·  📣  PeDRA poster, FDA talks begin

What three patients cannot prove

The pilot was designed to find a signal, and it found one. It was not designed to rule anything out. A single-arm study with three participants cannot separate drug effect from the natural history of a disease in which individual lesions vary in size and behavior. Lesion selection was left to investigators. Assessment was clinical and unblinded. The 30-day window says nothing about durability.

Safety looked mild at this scale. Twenty-two adverse events were reported across the three patients, all mild or moderate, all judged related to treatment and all resolved. The most common were redness and swelling at the injection site, each occurring five times in one patient. Flu-like illness and fever were the only events reported in more than one patient. No serious adverse events occurred.

Krystal’s stock moved up 2.97% to $330.69 on the day of the disclosure. Shares are up 33.9% year to date, against a 3% rise for the biomedical and genetics industry. Zacks still carries the stock at Rank #4, a sell, which is a reminder that a promising lesion count and a commercial franchise are different assets. Krystal already sells a gene therapy; the market is asking whether the pipeline can produce a second one.

What Krystal needs to prove next

The company says it has begun discussions with the FDA about advanced development and will update the market after alignment. The design of that trial will matter more than the pilot’s headline number. A randomized comparison against surgery, or against watchful waiting for selected lesions, would answer the question clinicians actually face: whether an injection can replace an operation.

The endpoint is the next hurdle. Lesion-level shrinkage is easy to measure and hard to price. Payers and surgeons care about operations avoided, scars prevented and quality of life gained over years, not weeks. Krystal will also need to show that the immune response is real and durable, either through biopsies of distant lesions or through T-cell assays that track reactivity against tumor antigens.

There is a manufacturing question underneath all of it. Redosable viral vectors are Krystal’s specialty, but each dose is a biologic that must be controlled, shipped and administered in a dermatology clinic. The economics improve if a single course prevents dozens of surgeries. They deteriorate if patients need indefinite re-treatment.

“The breadth and depth of responses reported today on landmark lesions located in various parts of the patient’s body showcase the systemic immune-activating potential of HSV-1 encoded IL-2 and IL-12.”— Suma Krishnan, President of Research and Development, Krystal Biotech

For now, the useful read is narrow and specific. A locally injected virus produced measurable shrinkage in most tracked tumors, including tumors it never touched, in a disease where repeated surgery is the standard of care. That is enough to justify a larger trial. It is not enough to call a treatment.