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Insulin Lispro is manufactured commercially in a manner quite similar to the ‘proinsulin’ route used to manufacture native recombinant human insulin. A synthetic gene coding for LysB28-ProB29 proinsulin is expressed in E. coli. Following fermentation and recovery, the proinsulin is treated with trypsin and carboxypeptidase B, resulting in the proteolytic excision of the engineered insulin molecule. It is then purified to homogeneity by a number of high-resolution chromatographic steps. The final product formulation also contains m-cresol (preservative and stabilizer), zinc oxide (stabilizer), glycerol (tonicity modifier) and a phosphate-based buffer. The commercial product has a shelf-life of 2 years when stored at 2-8°C.
The generation of engineered insulin analogues raises several important issues relating to product safety and efficacy. As mentioned in a general context in Chapter 3, alteration of a native protein’s amino acid sequence could render the engineered product immunogenic. Such an effect would be particularly significant in the case of insulin, as the product is generally administered daily for life. In addition, alteration in structure could have unintended (in addition to intended) influences upon pharmacokinetic and/or pharmacodynamic characteristics of the drug. Pre-clinical and, in particular, clinical evaluations undertaken upon the analogues thus far approved, however, have confirmed their safety as well as efficacy. The sequence changes introduced are relatively minor and do not seem to elicit an immunological response. Fortuitously, neither have the alterations made affected the ability of the insulin molecule to interact with the insulin receptor and trigger the resultant characteristic biological responses.
Figure 8.8. 3-D structure of the engineered fast-acting insulin, ‘Insulin Lispro’. Photo from Ciszak et al.
(1995), by courtesy of the Protein Data Bank: http://www.rcsb.org.pdb/
Additional means of insulin administration
Chapter 2 details recent progress in developing insulin formulations for delivery via the oral or pulmonary route. An additional approach, which may mimic more closely the normal changes in blood insulin levels, entails the use of infusion systems which constantly deliver insulin to the patient. The simplest design in this regard is termed an ‘open loop system’. This consists of an infusion pump which automatically infuses soluble insulin subcutaneously, via a catheter. Blood glucose levels are monitored manually and the infusion rate is programmed accordingly.
While the potential of such systems is obvious, they have not become popular in practice, mainly due to complications which can potentially arise, including:
• abscess formation or development of cellulitis at the site of injection;
• possible pump malfunction;
• catheter obstruction;
• hypersensitivity reactions to components of the system;
• requirement for manual blood glucose monitoring.
The closed-loop system (often termed the ‘artificial pancreas’) is essentially a more sophisticated version of the system described above. It consists not only of a pump and infusion device, but also an integral glucose sensor and computer, which analyses the blood glucose data obtained and adjusts the flow-rate accordingly. The true potential of such systems remains to be assessed.
HORMONES OF THERAPEUTIC INTEREST 321 Treating diabetics with insulin-producing cells
While infusion pumps can go some way towards mimicking normal control of blood insulin levels, transplantation of insulin-producing pancreatic cells should effectively ‘cure’ the diabetic patient. This approach has been adopted thus far with almost 200 patients, with encouraging results.
Initial experiments in the 1970s using inbred strains of rats illustrated the feasibility of this approach. Insulin-producing pancreatic islet cells ‘donated’ by one set of rats were transplanted into other rats of the same strain, first made diabetic by injection with drugs such as streptozotocin, which destroy the pancreatic B cells.
Such islet grafts permanently returned blood glucose levels in the diabetic animals to normal values. Even more encouraging, this treatment prevented development of diabetic-associated complications of kidney and eye function (which, in human diabetics, can lead to kidney failure and partial blindness).
The technique simply entails injecting the insulin-producing islets into the portal vein. These cells subsequently lodge in smaller vessels branching from the vein. Here, they can constantly monitor blood glucose levels and secrete insulin accordingly. About 400 000-800 000 islet B cells are usually transplanted. In most instances, these function for up to 3 years, although they often fail to control blood glucose levels fully, i.e. supplemental insulin injections are sometimes required.
The islet cells transplanted into humans are obtained from pancreatic tissue of deceased human donors (Figure 8.9). Implantation of these cells in recipients displaying a competent immune system would, at best, be of transient therapeutic benefit. The ensuing immune response would quickly destroy the foreign cells. Studies conducted thus far in humans have utilized diabetic patients who have received kidney transplants, as these are already subject to immunosuppressive therapy. However, a major stumbling block to the widespread adoption of this therapeutic approach is, predictably, the requirement to induce concurrent immune suppression.