Genetic medicine delivery needs a directed evolution

The low efficiency of conventional vectors used for genetic medicines often means delivering them at high doses and by suboptimal routes, leading to inflammation, toxic side effects and limited efficacy. David Kirn is co-founder and CEO of 4D Molecular Therapeutics (4DMT), a company using directed evolution to invent new adeno-associated virus (AAV) vectors that are tailored to specific target tissues and diseases. Here, he discusses the challenges involved in delivering genetic therapies, and how directed evolution is helping to overcome them.

What are the current challenges in genetic medicine?

The potential of genetic medicine is huge, especially because it offers a highly targeted approach. But there are still challenges to overcome. One challenge is delivery to the target cell types or tissues. A second challenge is transduction of the vector into the cell for transgene expression. A third challenge is inflammation and immunogenicity, especially if you need to use a high dose to overcome inefficient delivery. And a fourth challenge is that there are preexisting antibodies in the population for certain vectors, which can limit their use in genetic therapies.

How can these challenges be addressed?

We need better vectors. Current AAVs are already an improvement over first-generation vectors, but they’re not targeted to any particular tissue. What we really need are vectors that can be targeted to specific tissues or cell types using a routine route of administration so we can improve efficacy, and get the doses down to reduce immunogenicity and toxicities. Superior vectors also improve commercialization because the cost falls, and we can treat large disease populations.

How are you making targeted vectors?

We use a Nobel Prize-winning technology called directed evolution that has been used to invent optimized industrial enzymes and monoclonal antibodies, for example. We were the first team to apply directed evolution to AAVs at industrial scale. We start with a profile to describe the target tissues, administration route and dose range for the vector we want to invent. We then generate libraries of synthetic AAV capsids with about a billion vector sequences. We use competitive selection, both in vivo in primates and ex vivo in human tissue, to determine which AAV capsid best fits the target vector profile. For in vivo selection, we use the optimal route of administration. For example, we use intravitreal delivery to the retina and aerosol delivery to the lungs. The beauty of this approach is that the vectors we invent are modular. The genetic cargo can routinely be switched, so with a single vector we can efficiently build a portfolio of therapies for the same therapeutic area.

Which vectors have you developed in this way?

Through directed evolution we’ve invented a low-dose and well-tolerated intravitreal vector to transduce retina cells efficiently, an aerosol vector for delivery to airways that can penetrate through mucous barriers and resist antibodies, and low-dose intravenous vectors for delivery to the heart or central nervous system.

How will these new types of vector change genetic medicine?

Vectors invented with directed evolution have improved efficacy and safety profiles. These customized vectors are needed for numerous and diverse genetic approaches: gene editing, RNA interference, and targeted protein therapeutics, for example. All of these fields within genetic medicine will benefit, as will patients with severe common diseases.