Long-Acting Insulin Formulations
The normal human pancreas secretes approximately 1 unit of insulin (0.035 mg) per hour to maintain basal glycemic control. Adequate basal insulin levels are a critical component of diabetes therapy because they regulate hepatic glucose output, which is essential for energy production by the brain. Consequently, long-acting insulin formulation must provide a very different pharmacokinetic profile than “meal-time” insulin formulation.
There are three long-acting insulin preparations currently commercially available: Ultralente, which was developed in the 1950s and two insulin analogs, Lantus (insulin glargine) and Levemir (insulin detemir), which have been recently approved (Table 1; Fig. 1). Ultralente and Lantus derive theirprotracted time-action profiles from the slow and relatively constant dissolution of solid particles in the subcutaneous tissue. This slow dissolution precedes the dissociation of insulin into absorbable units, and thus the rate of absorption (units per hour) into the bloodstream is significantly decreased in comparison to that of a solution (mealtime) formulation. Levemir, on the other hand, achieves its protracted effect by a combination of structural interactions and physiolo-gical binding events (Havelund et al., 2004).
Ultralente is analogous to NPH insulin in that they are both formulated as crystalline insulin suspensions. However, the preparations differ in several key aspects. For one, under microscopic examination, the larger rhombohedral Ultralente microcrystals are notably different than the much smaller rod-shaped NPH microcrystals. This differ-ence originates from the different crystallization conditions employed to prepare these formulations as well as the excipients used. Ultralente contains no protamine and is crystallized at pH 5.5 in the presence of zinc, NaCl, and acetate buffer. The subsequent formulation process involves adjustment of the pH to a final value of 7.4, with the addition of excess zinc and methylparaben as an antimicrobial agent (pre-servative). The different formulation excipients used for Ultralente in comparison to NPH are a reflection of the different way in which the insulin molecules are complexed into the respective crystal lattices. NPH crystals are believed to be composed of zinc insulin hexamers stabilized as a complex with protamine and preservative molecules (Balschmidt et al., 1991), whereas, Ultralente crystals incorporate zinc insulin hexamers only (Brange, 1987a; Yip et al., 1998). A consequence of this composition is that methylpar-aben must be utilized as the preservative for Ultralente formulations because, unlike phenol and m-cresol, it does not interact with and destabilize the Ultralente crystal lattice.
As with all suspension products, Ultralente insulin should be uniformly resuspended prior to withdrawal of the dose from the vial to ensure accurate dosage. Ultralente has an onset of action of 0.5 to 3 hr, peak activity between 4 and 20 hr, and duration of action from 20 to 36 hr (Table 2). Similar to other insulin formulations, the variations in time-action are due to factors such as dose, site of injection, temperature, and the patient’s physical activity. Ultralente may be mixed with Regular insulin and Humalog, although its use in mixtures is constrained to extemporaneous mixing with immediate use for the reasons outlined for Lente. Much like the Lente insulin formulation use of Ultralente is declining
and its commercial manufacturing may soon cease. Insulin glargine (Lantus; GlyA21, ArgB31, ArgB32
human insulin; Sanofi-Aventis), is a long-acting insulinanalog, whose amino acid sequence modifications are highlighted in Table 1 and Figure 1. This analog differs from human insulin in that the amino acid asparagine is replaced with glycine at position A21 and two arginine residues have been added to the C-terminus of the B-chain. The impact of the additional arginine residues is to shift the pI from a pH of 5.4 to 6.7, thereby producing an insulin analog that is soluble at acidic pH values, but is less soluble at the neutral pH of subcutaneous tissue. Lantus is a solution formulation prepared under acidic conditions, pH 4.0. The intro-duction of glycine at position A21 yields a protein with acceptable chemical stability under acidic formulation conditions, since the native asparagine is susceptible to acid-mediated degradation and reduced potency. Thus, the changes to the molecular sequence of insulin have been made to improve chemical stability and to modulate absorption from the subcutaneous tissue, resulting in an analog that has approximately the same potency as human insulin. The Lantus formulation is a clear solution that incorporates zinc and m-cresol (preservative) at a pH value of 4. Consequently, Lantus does not need to be resuspended prior to dosing. Immediately following injection into the subcutaneous tissue, the insulin glargine precipitates due to the pH change, forming a slowly dissolving precipitate. This results in a relatively constant rate of absorption over 10.8 to 24 hr with no pronounced peak (Table 3). This profile allows once-daily dosing as a patient’s basal insulin. As with all insulin prepara-tions, the time course of Lantus may vary in different individuals or at different times in the same individual and the rate of absorption is dependent on blood supply, temperature, and the patient’s physical activ-ity. Lantus should not be diluted or mixed with any other solution or insulin, as will be discussed below.
Insulin detemir (Levemir; LysB29(N-tetradeca-noyl)des(B30)human insulin; Novo-Nordisk A/S) utilizes acylation of insulin with a fatty acid moiety as a means to achieve a protracted pharmacological effect. As shown in Table 1 and Figure 1, the B30 threonine residue of human insulin is eliminated in insulin detemir and a 14-carbon, myristoyl fatty acid is covalently attached to the e-amino group of LysB29. The analog forms a zinc hexamer at neutral pH in a preserved solution. Clinical studies have reported that insulin detemir displays lower pharmacokinetic and pharmacodynamic variability than NPH (Hermansen et al., 2001; Vague et al., 2003). An approximate description of the pharmacodynamic profile of Levemir is listed in Table 3. This analog appears to display a slower onset of action than NPH without a pronounced peak (Heinemann et al., 1999). However, whether the duration of the protracted effect can truly be considered sufficient enough to warrant classifica-tion of insulin detemir as a long-acting insulinremains a subject of debate since published clinical studies of this insulin analog are typically referenced to intermediate-acting NPH.
Binding of the tetradecanoyl-acylated insulin to albumin was originally proposed as the underlying mechanism behind the observed prolonged effect for the modified insulin analog; however, recent investi-gations on insulin detemir have determined that the mechanism is more complex (Havelund et al., 2004). It has been proposed that subcutaneous absorption is initially delayed as a result of hexamer stability and dihexamerization. Such interactions between hex-amers are a likely consequence of the symmetrical arrangement of fatty acid moieties around the outside of the hexamers (Whittingham et al., 2004), as shown by X-ray crystallographic studies. These associated forms further bind to albumin within the injection site depot. Additional prolongation may result due to albumin binding.