New Injection Site Dynamics of Insulin Lispro Discovered Through Photoacoustic Imaging

In a recent study, Khadria et al ¹ employed photoacoustic imaging to discover previously unknown injection site dynamics of insulin lispro and its formulation excipients. This work made 2 major advancements in the field of insulin, first, through studies on rodents and pigs, it showed that hexamer dissociation rate is not the absorption rate-limiting step for insulin lispro, and second, it characterized insulin diffusion at the injection site of a live animal for the first time. Rapid-acting insulins such as insulin lispro take ~15 minutes to get absorbed; however, this time frame is too long to match the postprandial glucose rise.^2,3 Since the last 20 years, it is widely believed that the hexamer-to-monomer dissociation rate is the absorption rate-limiting step for rapid-acting insulins. To make faster insulins, several studies reported developing stabilized monomeric insulins; however, they fail to decrease glucose at a faster rate than insulin lispro. ⁴

Khadria et al compared the injection site kinetics of dye-labeled insulin lispro in the Humalog and the recently Food and Drug Administration (FDA)-approved ultrafast Lyumjev formulations. The authors developed a high-resolution 3-dimensional photoacoustic imaging-based approach to quantify the absorption kinetics of the insulin formulations at the injection sites of mice ears. They showed that the insulin lispro disappearance from the injection site in the Lyumjev formulation is 2 times faster than in the Humalog formulation, which is similar to the results from clinical studies (Figure 1a). ⁵ By modeling the injection site absorption kinetics, the authors proved that the hexamer dissociation rate is not the absorption rate-limiting step for insulin lispro; but the rate at which it crosses the blood vessels determines its onset action time. They validated their imaging observation by studying unlabeled clinical quality insulin samples in 46 pigs, where they developed a stable monomeric version of insulin lispro and compared its pharmacokinetic profile with that of hexameric insulin lispro in Humalog formulation to observe a complete overlap for the first 2 hours (Figure 1b). These results prove incorrect a widely believed theory about injection site dynamics of rapid-acting insulins and provide a better approach to making faster insulins, that is, by increasing the microvascular transfer rate.

Figure 1.

Dynamics of insulin lispro in Humalog and Lyumjev formulations. (a) Injection site kinectics of dye-labeled insulin lispro in Humalog and Lyumjev formulations studied in the mouse ears through photoacoustic microscopy; PA, Photoacoustic. (b) Pharmacokinetics of unlabeled monomeric and hexameric insulin lispro in Humalog formulation, and hexameric insulin lispro in Lyumjev formulation studied in pigs. (c) Diffusion of dye-labeled insulin lispro in the Humalog formulation in the mouse ear. (d) Diffusion of dye-labeled insulin lispro in the Lyumjev formulation in the mouse ear. Figure reproduced from the results reported by Khadria et al. ¹

The authors further developed an imaging-based approach to characterize injection site insulin diffusion. They show that insulin lispro in the Lyumjev formulation diffuses up to three times faster than in the Humalog formulation, which plays a critical role in its faster absorption (Figures 1c and d). Until now, no other study characterized insulin diffusion at the injection site in vivo, and all the previous efforts were based on ex vivo tissues or phantoms. The approach developed by Khadria et al. can be applied to study a wide range of insulin formulations and other drugs to determine their initial onset action time, which will aid in the quicker development of life-saving drug therapeutics.

The methods and results reported by Khadria et al, help solve the crucial riddles around injection site dynamics of rapid-acting insulins. This study is a major step towards the development of faster and better-controlled insulins to help improve the lives of people living with Type 1 diabetes.