Insulin Aspart (NovoLog, Fiasp)

Insulin aspart was the second rapid-acting insulin analog ever approved, arriving in 2000 four years after insulin lispro with a different single amino-acid substitution that produces the same faster-absorption effect. It's also a real, ongoing case study in how the FDA's biosimilar pathway is finally reaching insulin two decades after this drug's original approval: 2025 alone brought both the first rapid-acting insulin biosimilar ever approved and, five months later, the first one designated interchangeable with its reference product.

A second rapid-acting analog, a different amino-acid trick

Four years after insulin lispro became the first approved insulin analog, Novo Nordisk brought a second, independently engineered rapid-acting analog to market: insulin aspart, marketed as NovoLog in the US (NovoRapid elsewhere) and FDA-approved in 2000 (NDA 020986). Where lispro reverses two amino acids at positions B28 and B29, insulin aspart makes a single substitution at position B28 — replacing the proline there with aspartic acid, a negatively charged amino acid whose charge disrupts the same hexamer-forming hydrophobic contacts lispro's swap disrupts, by a different structural route. The practical effect is functionally similar to lispro: faster dissociation into monomers after injection, and a correspondingly faster onset and shorter duration of action than regular human insulin, giving prescribers and patients a second, chemically distinct rapid-acting option rather than a copy of Lilly's drug.

The pivotal evidence

Insulin aspart's approval rested on a body of comparative trials against regular human insulin, the largest a six-month, multicenter, randomized, open-label trial across 88 centers in eight European countries, enrolling 1,070 adults with type 1 diabetes randomized 2:1 to mealtime insulin aspart or regular human insulin, both alongside NPH basal insulin (Home PD, Lindholm A, Riis A, for the European Insulin Aspart Study Group, "Insulin aspart vs. human insulin in the management of long-term blood glucose control in Type 1 diabetes mellitus: a randomised controlled trial," Diabet Med 2000;17(11):762-770, PMID 11131100).¹ The trial found comparable or modestly improved long-term glycemic control (HbA1c) with insulin aspart against regular human insulin, together with better post-meal glucose profiles and a lower rate of major hypoglycemic episodes — the same broad pattern of faster, more meal-matched action already established for lispro, replicated independently in aspart's own dedicated trial program.

Fiasp: a faster-absorbing reformulation of the same molecule

On 29 September 2017, the FDA approved Fiasp (insulin aspart injection, faster-acting formulation), a reformulation of the same insulin aspart molecule with two added excipients — niacinamide (vitamin B3), which speeds the drug's initial absorption, and L-arginine, added for stability. Trials comparing the two found Fiasp's insulin appearing in the bloodstream roughly twice as fast as standard NovoLog's, and its label permits dosing either at the start of a meal or up to 20 minutes after starting to eat — a real, evidence-backed timing flexibility that NovoLog's label doesn't offer. Fiasp's FDA approval was later extended to insulin pump use and, in 2020, to pediatric patients, but it remains the same active molecule as NovoLog with a different delivery-speed formulation, not a third distinct insulin analog.

2025: the first insulin biosimilar, then the first interchangeable one

Unlike insulin lispro and insulin glargine, whose lower-cost competition (Admelog, Basaglar) arrived through pathways that predate the FDA's modern biosimilar framework, insulin aspart is the first insulin on this site with real biosimilar competition approved under that framework. On 14 February 2025, the FDA approved Merilog (insulin aspart-szjj, Sanofi) as a biosimilar to NovoLog — the first rapid-acting insulin biosimilar the agency had ever approved, though not designated interchangeable, meaning a pharmacist generally can't substitute it without the prescriber specifying it. Five months later, on 15 July 2025, the FDA approved Kirsty (insulin aspart-xjhz, Biocon Biologics) as an interchangeable biosimilar to NovoLog — the first rapid-acting insulin biosimilar of any kind to earn that stricter designation, which does allow pharmacy-level substitution the way this site's Insulin page describes for Semglee, the first interchangeable insulin glargine biosimilar, approved in 2021.

Current status

NovoLog and Fiasp remain actively marketed by Novo Nordisk, alongside Sanofi's Merilog and Biocon Biologics' interchangeable Kirsty. As described on this site's Insulin page, NovoLog is also one of the specific products named in the 2025-2026 "most-favored-nation" manufacturer pricing agreements running through the federal TrumpRx platform — insulin aspart's pricing and competitive landscape is, as a result of the 2025 biosimilar approvals, genuinely more crowded now than at any point since its original 2000 approval.