The premise of Gene therapy relies on introducing genetic materials into patients to augment, alter, or correct gene expression thereby providing a one-time curative treatment for many debilitating diseases. Adeno-associated virus (AAV) vectors are the leading platform for in vivo gene delivery for the treatment of a variety of monogenic human diseases. A non-pathogenic parvovirus, AAV was first identified as a contaminant in adenovirus preparation for electron microscopy. AAV is small (25 nm), non-enveloped parvovirus carrying a single-stranded DNA genome of around 4.7 kilobases that is encased within a protein shell (kb). AAV belongs to the genus dependovirus and requires co-infection with other viruses, primarily adenoviruses, to replicate. 94% of the genome along with all viral gene sequences are removed to make way for the insertion of the therapeutic transgene along with regulatory elements to achieve tissue specific therapeutic protein expression. Once in the nucleus, the vector genome remains mostly episomal and any integration into host genome is inefficiently further reducing the risk of insertional mutagenesis.
These desirable characteristics have propelled AAV to become the preferred vector for in vivo Gene therapy treatments for numerous genetic disorders. Three AAV gene therapy drugs have since been authorized for the treatment of dyslipidemia, retinal blindness, and spinal muscular atrophy. The increase of active AAV clinical trials is due to efficacy data from multiple ongoing clinical trials as well as accomplishments seen with these authorized medicines. The development of AAV drugs depends on choosing an acceptable AAV capsid with the requisite characteristics for transgene delivery.
AAV is a naturally occurring human pathogen and a subset of people are likely to have been exposed to wild-type AAVs and thus have pre-existing adaptive immunity, including neutralizing antibodies (NAbs) and T cells against some AAVs. While NAbs can bind and neutralizing infused AAV particles to prevent cell entry, T cells eliminate any transduced cells thereby undermining clinical efficacy. Indeed, AAV seropositivity is an exclusion criteria in AAV gene therapy trials. NAb seroprevalence is estimated in a population to understand the percentage of patients who may be excluded from receiving AAV gene therapy. A new research article published in the journal Human Gene Therapy by Pfizer researchers: Dr. Arpana Khatri, Dr. Rajani Shelke, Dr. Shunjie Guan, and Dr. Suryanarayan Somanathan evaluated AAV NAbs against a panel of vectors among healthy donors in a single geographical location within the United States. Their results show a reduced seroprevalence (<50%) against the majority of AAV capsids. Interestingly, their data also shows that minority Black and Hispanic subjects had much higher AAV NAb incidence compared to whites, against several AAVs. Even when the overall seroprevalence was low, there was still a bias in increased incidence of NAb among minority subjects.
As the name implies, NAb is a measure of antibodies that neutralize AAVs and prevent their uptake and expression in cells. An orthogonal binding antibody assay only measures antibodies that bind to AAV (BAb). In general, BAb positive subjects outnumbered NAb positive subjects. For instance, twice as many donors had AAV5 BAbs than NAbs (60 percent vs. 30 percent, respectively). Further research shows cross-reactivity against AAVs from various clades, and donors with high antibody titers primarily cross-neutralized different AAVs. The finding that Black and Hispanic donors had higher NAb seroprevalence against most AAVs was the most intriguing finding from their study. These racial inequalities were more pronounced in male donors than in female donors. There were greater intraracial variations between the sexes, with white women and Hispanic men being more seropositive than their counterparts. These findings were verified in a second distinct donor cohort from the same region that also demonstrated greater incidence of NAb among minority donors.
Researchers suggest considering AAV seroprevalence in the patient groups when choosing capsids for drug development while minimizing sampling bias. The new study reveals a previously unrecognized variation in AAV NAb incidence by race that may have an important impact on seroprevalence estimations. In a statement to Medicine Innovates, Dr. Suryanarayan Somanathan said “The finding of higher seroprevalence among minorities was unexpected and intriguing, including the differences among races by gender. It would be interesting whether these observations hold true for other geographical locations in the US and the rest of the world”.
Khatri A, Shelke R, Guan S, Somanathan S. Higher Seroprevalence of Anti-Adeno-Associated Viral Vector Neutralizing Antibodies Among Racial Minorities in the United States. Human gene therapy. 2022 Apr 1;33(7-8):442-50.