# Development and Preclinical Testing of a Novel Neurodenervant in the Rat: C3 Transferase Mitigates Botulinum Toxin’s Adverse Effects on Muscle Mechanics

## TL;DR

A new combination of C3 transferase and botulinum toxin reduces muscle stiffness without harmful side effects in rats.

## Contribution

A novel neurodenervant formula combining C3 transferase with botulinum toxin is developed and tested.

## Key findings

- C3 transferase combined with BTX-A reduced active muscle forces by 43.5% without affecting passive forces or length range.
- C3 transferase alone did not cause muscle atrophy or increase collagen content.
- The combination eliminates BTX-A's adverse effects on extracellular matrix stiffening.

## Abstract

Spasticity, characterized by elevated muscle tone, is commonly managed with botulinum toxin type A (BTX-A). However, BTX-A can paradoxically increase passive muscle forces, narrow muscles’ length range of force exertion (lrange), and elevate extracellular matrix (ECM) stiffness. C3 transferase, known to inhibit myofibroblast and fascial tissue contractility, may counteract ECM stiffening. This study investigated whether combining BTX-A with C3 transferase reduces active forces without altering passive forces or lrange. Additionally, we examined the isolated effects of C3 transferase on muscle levels. Male Wistar rats received injections into the tibialis anterior (TA): Control (n = 7, saline) and C3 + BTX-A (n = 7, 2.5 µg C3 + 0.1U BTX-A). TA forces were measured one month post-injection, and isolated C3 transferase effects were assessed in separate groups (Control and C3, n = 6 each). Active forces were 43.5% lower in the C3 + BTX-A group compared to the Control group. No differences between groups in passive forces (p = 0.33) or lrange (p = 0.19) were observed. C3 transferase alone had no significant effect on relative muscle mass (p = 0.298) or collagen content (p = 0.093). Supplementing BTX-A with C3 transferase eliminates BTX-A’s adverse effects at the muscle level. C3 transferase alone causes no atrophy or collagen increase, which are key factors in BTX-A-induced ECM stiffening. This novel neurodenervant formula shows promise for advancing spasticity management.