A better way to spray? – a model-based optimization of nasal spray use protocols

Drug delivery for viral infections, such as SARS-CoV-2, can be enhanced significantly by targeting the nasopharynx, which is the dominant initial infection site in the upper airway, for example by nasal sprays. However, under the standard protocol ("current use" or CU), the nozzle enters the nose almost vertically, resulting in suboptimal deposition of drug droplets at the nasopharynx. Using computational fluid dynamics (CFD) simulations in four anatomic nasal geometries, we propose an "improved use" (IU) protocol. It entails pointing the spray bottle at a shallower angle (almost horizontally), aiming slightly towards the cheeks. We have simulated the performance of this protocol for conically injected spray droplet sizes of 1–24 µm at two breathing rates: 15, 30 L/min. The lower flowrate is for resting breathing and follows a viscous-laminar model; the higher rate is turbulent and is tracked via Large Eddy Simulation. Experimentally-validated CFD results show that targeted delivery via IU outperforms CU by over 2 orders of magnitude, for both flowrates. Improved delivery by IU is robust to small changes in spray direction, underlining the practical utility of this simple change in administration protocol.