AAV Tropism for Muscle Tissues

In muscle gene therapy, the goal is often to achieve widespread (systemic) transduction to reach every skeletal muscle in the body, as well as the heart and the diaphragm.

While AAV9 and AAV8 were long considered the primary choices, AAVrh74 has emerged as a clinical leader due to its high efficiency and lower pre-existing immunity in humans.

Serotype	Primary Target	Transduction Efficiency	Best Use Case	Key Features
AAVrh74	Skeletal & Cardiac	Ultra High	Muscular Dystrophy (DMD)	Used in the FDA-approved Elevidys. Exceptionally potent for whole-body muscle delivery.
AAV9	Cardiac & Skeletal	Very High	Heart & Neuromuscular	The most versatile natural serotype; crosses the blood-brain barrier and the vascular wall.
AAV8	Skeletal & Liver	High	Systemic delivery	Robust expression in skeletal muscle, but often shows high “leakage” into the liver.
AAV6	Skeletal Muscle	High	Local intramuscular	A natural hybrid (AAV1/AAV2) that excels at entering myofibers; often used in local injections.
AAV1	Skeletal Muscle	High	Localized muscle	One of the first serotypes used for muscle; very efficient but has high human pre-existing immunity.
AAV7	Skeletal Muscle	Moderate	NHP research	Originally isolated from rhesus macaques; similar to AAV1 and AAV8 in muscle efficiency.
MyoAAV	Skeletal Muscle	Ultra High	Synthetic / Muscle-specific	An engineered capsid that targets muscle 10–50x better than AAV9 while avoiding the liver.

Delivery Methods for Muscle

Muscle makes up roughly 40% of human body mass, so delivery strategy is as important as the serotype:

1. Systemic (Intravenous – IV): Essential for diseases like Duchenne (DMD) or Pompe, where every muscle needs treatment. AAVrh74 and AAV9 are the top choices here.
2. Intramuscular (IM): Direct injection into a specific muscle (e.g., the bicep or tibialis). Best for localized issues or using the muscle as a “factory” to secrete proteins into the blood. AAV1 and AAV6 are very effective for this.
3. Regional Limb Perfusion: Blocking blood flow in a limb and “flooding” it with the virus. This allows for high concentration in the leg or arm while minimizing the dose to the liver.

Muscle-Specific Promoters

To prevent the virus from expressing your gene in the liver (where most IV-injected AAVs end up), researchers use promoters that only “turn on” inside muscle cells:

• MCK (Muscle Creatine Kinase): The classic promoter for skeletal and cardiac muscle. Often used in its truncated versions (like dMCK or tMCK) to save space.
• MHCK7: A powerful synthetic promoter that combines elements of MCK and alpha-MHC to drive high expression in both skeletal muscle and the heart.
• cTnT (Cardiac Troponin T): If you only want to target the heart and avoid skeletal muscle.
• Desmin: Targets both types of striated muscle (skeletal and cardiac) but is slightly less specific than MCK.

A Note on “MyoAAV”

If you are doing cutting-edge research, you should look into MyoAAV. This is a 2025-standard synthetic capsid developed via “directed evolution.” It is designed to bind specifically to an integrin (αvβ6) found on muscle cells, allowing for much lower doses than AAV9 while significantly reducing liver toxicity.