Executive Summary

The architecture of a viral vector plasmid is a primary determinant of therapeutic efficacy and manufacturability. Engineering the inverted terminal repeats (ITRs), promoter, and polyadenylation (PolyA) signal within the transgene cassette directly impacts vector packaging efficiency, tissue-specific expression, and the durability of the clinical effect. An optimized plasmid design minimizes downstream risks during IND-enabling toxicology studies and later-phase manufacturing.

The Plasmid Backbone: Engineering for AAV Production

The plasmid serves as the foundational blueprint for a viral vector. For AAV production, a triple transfection method in HEK293 cells is common, requiring three distinct plasmids: a plasmid containing the gene of interest flanked by ITRs, a helper plasmid providing Adenovirus genes, and a plasmid encoding the AAV replication (Rep) and capsid (Cap) proteins. The design of the ITR-containing plasmid is a point of significant leverage for program success.

The integrity of the ITR sequences is a known variable in production yield. Unstable or truncated ITRs can lead to inefficient packaging and a higher proportion of empty capsids, complicating downstream purification and characterization.

Selecting Promoters and Transgenes for Targeted Expression

The choice of promoter and the design of the transgene cassette are tailored to the specific therapeutic indication. The goal is to achieve sufficient, durable expression in the target tissue. As demonstrated in preclinical evaluations, different AAV serotypes exhibit varied tropism and expression kinetics across tissues (PMID: 22849678, 23659250). The promoter must work in concert with the selected capsid to achieve the desired biological outcome.

Key considerations for cassette design include:

• Promoter Strength and Specificity: Balancing high-level expression with the need to avoid immune responses or off-target effects.

• Codon Optimization: Modifying the transgene sequence to match the codon usage of human cells can enhance translation efficiency without altering the final protein sequence.

• Introns and Enhancers: The inclusion of specific genetic elements, such as introns, can augment gene expression levels in certain contexts.

We started collaborating with UPenn Vector core in 2023 and the AAV vector which they manufactured laid a foundation for development of a gene therapy candidate which will enter soon preclinical studies. The key people from UPenn Vector Core joined Franklin Biolabs and our partnership transitioned without interruption from UPenn Vecor Core to Franklin Biolabs Research Vector Division. The Franklin Biolabs team is integral to our AAV-vector based gene therapy candidate development, and we hope to continue this productive collaboration for years to come.
— Biotech Partner

Polyadenylation Signals and Transcript Stability

The final element in the cassette, the PolyA signal, contributes to producing a stable and translatable transcript. While common signals like the bovine growth hormone (bGH) polyA or SV40 polyA are effective, the selection can influence expression levels. A weak or inefficient PolyA signal can result in transcript degradation, lowering the ultimate yield of the therapeutic protein and compromising potency.

The entire cassette, from the 5′ ITR to the 3′ PolyA signal, must be contained within the AAV packaging limit of approximately 4.7 kilobases. This constraint requires precise engineering to include all necessary elements without sacrificing function.