Discovery and Development
Insulin lispro was one of the earliest commercially available fast-acting insulin analogue developed as an alternative to regular human insulin for the treatment of diabetes. It was discovered and developed by Eli Lilly and Company in the early 1990s after scientists found that they could alter the amino acid sequence of natural human insulin to produce analogues with altered pharmacokinetic and pharmacodynamic profiles. Admelog differs from regular human insulin by the reversal of the proline and lysine residues at positions B28 and B29 on the insulin molecule. This small but crucial change allows Admelog to dissolve and act faster in the body compared to regular human insulin.

Clinical Trials and Approval
Eli Lilly conducted extensive preclinical and clinical trials in the 1990s to evaluate the safety, efficacy and pharmacokinetic properties of Admelog compared to regular human insulin. The landmark clinical trials established that Admelog starts working faster and has a shorter duration of action compared to regular insulin. It also showed better post-prandial glucose control with Admelog which can help reduce the risk of long-term diabetes complications. Based on the positive results from Phase 3 clinical trials involving over 1000 patients with type 1 and type 2 diabetes, the FDA approved Admelog under the brand name Humalog in 1996, making it the first rapid-acting insulin analogue available in the US market.

Dosage Forms and Delivery Methods
Initially, Admelog was only available as a clear solution for injection using syringes. Over the years, Eli Lilly developed newer delivery methods and dosage forms to improve treatment flexibility and patient experience with Admelog therapy. Pre-filled Admelog pens were launched to facilitate self-administration. Most recently, Admelog is also available in a formulation combined with Admelog protamine that can be administered as injections before meals (lispro) and at bedtime (lispro protamine). Insulin pump therapy using continuous subcutaneous infusion of Admelog further enhanced glycemic control for pump users.

Market Performance and Competition
Since its market launch in 1996, insulin lispro has garnered significant market share and grown to be one of the top-selling diabetes drugs globally. It captured over 30% of the fast-acting insulin market by the late 1990s. However, it faced substantial competition over the years from similar rapid-acting insulin analogues like insulin aspart (Novolog) and insulin glulisine (Apidra) launched by competitor companies in the 2000s. The entrance of biosimilar versions of Admelog in recent years has further increased competition. Still, Admelog maintains its leadership position due to continued innovations, established brand affinity among patients and physicians, and Eli Lilly's marketing efforts over two decades.

Intellectual Property and Generic Versions
Eli Lilly invested heavily in developing a strong patent portfolio around the discovery, composition and formulations of Admelog during the product's development phase in the 1990s. The company secured several composition of matter patents covering the modified Admelog molecule that provided exclusivity in markets until 2014. However, the patents have expired now paving the way for competing drugmakers to seek regulatory approvals for biosimilar versions. In recent years, a few companies have received FDA approval to market Admelog biosimilar products with Eli Lilly slashing prices of Humalog to counter potential revenue losses owing to competition. Nevertheless, Lilly's Humalog franchise still holds a large profitable share in the lucrative diabetes drug market.

Future Directions
Despite facing competition from copies now, Eli Lilly and collaborators continue to develop novel innovations around insulin lispro therapy. The company is involved in research on ultra-rapid formulations that work even faster than current versions. Combination therapies of Admelog with other anti-diabetic drugs are also in clinical testing phases. Digital health solutions integrating insulin pumps and glucose sensors with AI/machine learning systems could help optimize Admelog therapy based on real-time data in the coming years. Overall, Admelog as a fast-acting insulin still has promising potential to benefit millions of diabetes patients globally with emerging advanced treatment modalities.