Adeno-associated virus (AAV)-based systems are ideally suited for delivering gene therapy. After the viral vector infects the cell, the DNA doesn’t integrate into the host genome, instead existing primarily in an episomal form, which allows for long-term expression in nonreplicating cells without DNA damage to the host cells and the risk of inducing malignancy.
But introducing virus into patients can trigger an immune response, which multiple studies have reported after the virus was injected into the muscle, liver or intravenously. Injections into the eye and brain don’t seem to have the same immune response.
When an immune response is observed, investigators often use immunosuppressive drugs or steroids to keep the immune system from attacking the virus.
Terence Flotte and colleagues saw a similar immune reaction – development of antibodies to the viral proteins and a T-cell response – when patients with alpha-1 antitrypsin (AAT) deficiency were injected with an AAV-based therapy containing wild-type AAT developed by Applied Genetic Technologies Corp. (AGTC).
There was initially a modest decline in expression after the immune response, but the investigators decided not to treat with immunosuppressive drugs or steroids.
When investigators took muscle biopsies a year after the initial treatment, what they found was “very surprising,” according to Flotte, dean of the School of Medicine and provost and executive deputy chancellor of the University of Massachusetts Medical School.
“Over time, expression levels didn’t continue to decline,” Flotte told BioWorld Insight.
Not only was there protein expression, but the immune system didn’t seem to be clearing the virus. The muscle biopsies had intact AAV capsid protein a year after injection.
Flotte and colleagues determined that part of the immune response was coming from regulatory T cells that were modulating the immune response.
“The regulatory T-cell response enables a tolerance to develop,” Flotte said.
While the tolerance is good news for gene therapy, it isn’t clear how widespread it might be. There are multiple serotypes of the AAV, for instance, and it isn’t known whether they’ll induce the same regulatory T-cell response. It’s also unclear whether the response is specific to intramuscular injections and how the regulatory T-cell response might affect multiple injections.
Glybera (alipogene tiparvovec, Uniqure BV), the first AAV-based gene therapy approved in Europe, is administered through a one-time series of small intramuscular injection in the legs to treat the inherited disorder lipoprotein lipase deficiency. (See BioWorld Today, Nov. 5, 2012.)
For patients with AAT deficiency, which currently is treated with weekly infusions of AAT protein purified from donated human plasma that are in short supply, higher expression of AAT will be necessary to correct the inherited genetic defect. In the Phase IIa trial, the expression after one year was just 3 percent of the therapeutic target.
AGTC, of Gainesville Fla., plans to begin a Phase IIb trial next year using a different dosing methodology to increase the expression level of the AAT protein.
The firm has four additional gene therapy programs including a partnership with Sanofi SA’s Cambridge, Mass.-based Genzyme Corp. to develop a gene therapy product for wet age-related macular degeneration using an AAV vector that expresses sFLT01, an anti-VEGF fusion protein.