Four years ago, a small Philadelphia biotech company won U.S. approval for the first gene therapy to treat an inherited disease, a landmark after decades of research aimed at finding ways to correct errors in DNA.

Since then, most of the world's largest pharmaceutical companies have invested in gene therapy, as well as cell therapies that rely on genetic modification. Dozens of new biotech companies have launched, while scientists have taken forward breakthroughs in gene editing science to open up new treatment possibilities.

But the confidence brought on by such advances has also been tempered by safety setbacks and clinical trial results that fell short of expectations. In 2022, the outlook for the field remains bright, but companies face critical questions that could shape whether, and how soon, new genetic medicines reach patients. Here are five:

Food and Drug Administration approval of Spark Therapeutics' blindness treatment Luxturna a first in the U.S. came in 2017. A year and a half later, Novartis' spinal muscular atrophy therapy Zolgensma won a landmark OK.

But none have reached market since, with treatments from BioMarin Pharmaceutical and Bluebird bio unexpectedly derailed or delayed.

That could change in 2022. Two of Bluebird's treatments, for the blood disease beta thalassemia and a rare brain disorder, are now under review by the FDA, with target decision dates in May and June. BioMarin, after obtaining more data for its hemophilia A gene therapy, plans to soon approach the FDA about resubmitting an application for approval.

Others, such as CSL Behring and PTC Therapeutics, are also currently planning to file their experimental gene therapies with the FDA in 2022.

Approvals, should they come, could provide important validation for their makers and expand the number of patients for whom genetic medicines are an option. In biotech, though, approvals aren't the end of the road, but rather the mark of a sometimes challenging transition from research to commercial operations. With price tags expected to be high, and still outstanding questions around safety and long-term benefit, new gene therapies may prove difficult to sell.

A record $20 billion flowed into gene and cell therapy developers in 2020, significantly eclipsing the previous high-water mark set in 2018.

Last year, the bar was set higher still, with a total of $23 billion invested in the sector, according to figures compiled by the Alliance for Regenerative Medicine. About half of that funding went toward gene therapy developers specifically, with a similar share going to cell-based immunotherapy makers.

Driving the jump was a sharp increase in the amount of venture funding, which rose 73% to total nearly $10 billion, per ARM. Initial public offerings also helped, with sixteen new startups raising at least $50 million on U.S. markets.

Entering 2022, the question facing the field is whether those record numbers will continue. Biotech as a whole slumped into the end of last year, with shares of many companies falling amid a broader investment pullback. Gene therapy developers, a number of which had notable safety concerns crop up over 2021, were hit particularly hard.

Moreover, many startups that jumped to public markets hadn't yet begun clinical trials roughly half of the 29 gene and cell therapy companies that IPO'd over the past two years were preclinical, according to data compiled by BioPharma Dive. That can set high expectations companies will be hard pressed to meet.

Generation Bio, for example, raised $200 million in June 2020 with a pipeline of preclinical gene therapies for rare diseases of the liver and eye. Unexpected findings in animal studies, however, sank company shares by nearly 60% last December.

Still, the pace of progress in gene and cell therapy is fast. The potential is vast, too, which could continue to support high levels of investment.

"I think fundamentally, investment in this sector is driven by scientific advances, and clinical events and milestones," said Janet Lambert, ARM's CEO, in an interview. "And I think we see those in 2022."

The potential of replacing or editing faulty genes has been clear for decades. How to do so safely has been much less certain, and concerns on that front have set back the field several times.

"Safety, safety and safety are the first three top-of-mind risks," said Luca Issi, an analyst at RBC Capital Markets, in an interview.

Researchers have spent years making the technology that underpins gene therapy safer and now have a much better understanding of the tools at their disposal. But as dozens of companies push into clinical trials, a number of them have run into safety problems that raise crucial questions for investigators.

In trials run by Audentes Therapeutics and by Pfizer (in separate diseases), study volunteers have tragically died for reasons that aren't fully understood. UniQure, Bluebird bio and, most recently, Allogene Therapeutics have reported cases of cancer or worrisome genetic abnormalities that triggered study halts and investigations.

While the treatments being tested were later cleared in the three latter cases, the FDA was sufficiently alarmed to convene a panel of outside experts to review potential safety risks last fall. (Bluebird recently disclosed a new hold in a study of its sickle cell gene therapy due to a patient developing chronic anemia.)

The meeting was welcomed by some in the industry, who hope to work with the FDA to better detail known risks and how to avoid them in testing.

"[There's] nothing better than getting people together and talking about your struggles, and having FDA participate in that," said Ken Mills, CEO of gene therapy developer Regenxbio, in an interview. "The biggest benefit probably is for the new and emerging teams and people and companies that are coming into this space."

Safety scares and setbacks are likely to happen again, as more companies launch additional clinical trials. The FDA, as the recent meeting and clinical holds have shown, appears to be carefully weighing the potential risks to patients.

But, notably, there hasn't been a pullback from pursuing further research, as has happened in the past. Different technologies and diseases present different risks, which regulators, companies and the patient community are recognizing.

"We're by definition pushing the scientific envelope, and patients that we seek to treat often have few or no other treatment options," said ARM's Lambert.

Last June, Intellia Therapeutics disclosed early results from a study that offered the first clinical evidence CRISPR gene editing could be done safely and effectively inside the body.

The data were a major milestone for a technology that's dramatically expanded the possibility for editing DNA to treat disease. But the first glimpse left many important questions unanswered, not least of which are how long the reported effects might last and whether they'll drive the kind of dramatic clinical benefit gene editing promises.

Intellia is set to give an update on the study this quarter, which will start to give a better sense of how patients are faring. Later in the year the company is expecting to have preliminary data from an early study of another "in vivo" gene editing treatment.

In vivo gene editing is seen as a simpler approach that could work in more diseases than treatments that rely on stem cells extracted from each patient. But it's also potentially riskier, with the editing of DNA taking place inside the body rather than in a laboratory.

Areas like the eye, which is protected from some of the body's immune responses, have been a common first in vivo target by companies like Editas Medicine. But Intellia and others are targeting other tissues like the liver, muscle and lungs.

Later this year, Verve Therapeutics, a company that uses a more precise form of gene editing called base editing, plans to treat the first patient with an in vivo treatment for heart disease (which targets a gene expressed in the liver.)

"The future of gene editing is in vivo," said RBC's Issi. His view seems to be shared by Pfizer, which on Monday announced a $300 million research deal with Beam Therapeutics to pursue in vivo gene editing targets in the liver, muscle and central nervous system.

With more and more cell and gene therapy companies launching, the pipeline of would-be therapies has grown rapidly, as has the number of clinical trials being launched.

Yet, many companies are exploring similar approaches for the same diseases, resulting in drug pipelines that mirror each other. A September 2021 report from investment bank Piper Sandler found 21 gene therapy programs aimed at hemophilia A, 19 targeting Duchenne muscular dystrophy and 18 going after sickle cell disease.

In gene editing, Intellia, Editas, Beam and CRISPR Therapeutics are all developing treatments for sickle cell disease, with CRISPR the furthest along.

As programs advance and begin to deliver more clinical data, companies may be forced into making hard choices.

"[W]e think investors will place greater scrutiny as programs enter the clinic and certain rare diseases are disproportionately pursued," analysts at Stifel wrote in a recent note to investors, citing Fabry disease and hemophilia in particular.

This January, for example, Cambridge, Massachusetts-based Avrobio stopped work on a treatment for Fabry that was, until that point, the company's lead candidate. The decision was triggered by unexpected findings that looked different than earlier study results, but Avrobio also cited "multiple challenging regulatory and market dynamics."

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5 questions facing gene therapy in 2022 - BioPharma Dive