Subcutaneous (SC) injections

 

I used to wonder why subcutaneous (SC) injections were limited to ~1 mL or less. The maximum volume of SubQ injection that I worked in the small molecule segment is a 3 mL injection used for treatment of acute angioedima condition. During an interaction with my previous colleague I understand that one of the new innovation in combination drug products are 5 mL syringe barrels with Autoinjector compatibility for SubQ injection application. He also explained the role of an enzyme which helps for large volume SubQ injections. The reason for ~1 mL is, subcutaneous space isn’t an empty cavity it’s a dense, gel-like matrix made of collagen fibers embedded in Hyaluronic acid (hyaluronan). This structure restricts fluid spread. Injected liquid stays localized that builds pressure quickly. Injection volumes >1 mL causes tissue distension. This activates nociceptors, leading to pain and burning sensation. This structure also slows absorption. large boluses linger, causing swelling and discomfort. Because of this, traditional SubQ injections were typically limited to 0.5–1 mL, especially for self-administration formats like prefilled syringes and autoinjectors. The paradigm changed with the introduction of Hyaluronidase, an enzyme that temporarily modifies the SC tissue structure. Hyaluronidase works by: a) Breaking down hyaluronic acid chains b) Reducing the viscosity of the extracellular matrix c) Increasing tissue permeability (often called “spreading effect”) This effectively converts the SC space from a tight gel to more fluid, permeable environment, allowing injected drugs to spread over a larger area, Reduce local pressure and Be absorbed more rapidly Hyaluronidase was first identified in animal tissues. A major leap came with recombinant human hyaluronidase, rHuPH20. This is developed using gene expression systems to improve safety and consistency enabled co-formulation with biologics Approval of co-formulated monoclonal antibodies with hyaluronidase, allowing large-volume SC delivery (2–5 mL and beyond) instead of IV infusion. This enabled “large-volume subcutaneous (LVSC)” delivery, a major shift in biologics administration. This replaced long IV infusions with 5–8 minute SC injections. Hyaluronidase has fundamentally enabled patient-centric care (home administration vs hospital IV). This allowed high-dose biologics in SC format. This helped innovation in development of 5 mL syringe barrels with handheld autoinjectors and Large-volume wearable injectors. Fun fact: Hyluronidage (PH20) is present in sperm cells which will help to penetrate the cumulus–oocyte complex (the protective layer around the egg) by digesting hyaluronic acid.