Clinical endpoints of needle-free jet injector treatment: An in depth understanding of immediate skin responses

Objectives: Needle-free jet injectors have been used in dermatological practice for many years. However, predefined clinical endpoints that guide physicians to choose optimal device settings have not been clearly defined. Here, we evaluate immediate skin responses as clinical endpoints for needle-free jet injector treatments. Methods: We injected methylene blue in ex vivo human skin using an electronically-controllable pneumatic injector (EPI; 3–6 bar, 50–130 µl; n = 63), and a spring-loaded jet injector (SLI) with fixed settings (100 µl; n = 9). We measured the immediate skin papule (3D-camera), residual surface fluid (pipette), dermal dye distribution by estimating depth and width, and subcutaneous dye deposition. Results: EPI with 4 bar and 100 µl resulted in the largest skin papule of 48.7 mm³ (35.4–62.6 mm³) and widest dermal distribution of 8.0 mm (5.5–9.0 mm) compared to EPI with 6 bar and 100 µl (p < 0.001, p = 0.018, respectively). The skin papule volume showed a significant moderate to high positive correlation with the width and depth of dye distribution in the dermis (r_s = 0.63, r_s = 0.58, respectively; p < 0.001 for both correlations). SLI showed high variability for all outcome measures. Finally, a trend was observed that a small skin papule (≤7 mm) and little residual surface fluid (≤10% of injection volume) were warning signs for subcutaneous deposition. Conclusions: The immediate skin papule and residual surface fluid correspond with dermal drug deposition and are relevant clinical endpoints for needle-free jet injector treatments in dermatological practice.