After the discovery of the potent carcinogenic effect of AspB10 in rats the European Medicines Agency issued a detailed guidance on what should be done to exclude or minimize the carcinogenic hazard for new IAs. In the "Points to consider document on the non-clinical assessment of the carcinogenic potential of insulin analogues" [
4]
in vitro and
in vivo studies are described that may be appropriate to address the possible carcinogenicity of a new IA. In this document, an IA-induced increase in cell proliferation mediated by cross-reactivity with insulin-like growth factor 1 (
IGF-1) at the IGF1 receptor (IGF1R) was considered the most likely mechanism. The statement: "...
insulins with an increased mitogenic effect in comparison to the native human insulin in current use would constitute a major public health concern" makes clear that this hazard of IAs is taken most seriously. From today's perspective, almost ten years after adoption of the guidance document, this position may seem over-cautious. It is uncertain whether a new IA with such effects on cell proliferation could today or in the future get a positive recommendation for marketing authorization by the European Medicines Agency's Committee for Medicinal Products for Human Use. For IAs already licensed, however, effects on proliferation that have been seen in non-clinical experiments in combination with ultimately inconclusive epidemiological studies on the occurrence of cancer in humans were not considered sufficient for a suspension or withdrawal of the European marketing authorization by the European Medicines Agency. Insulin glargine has been shown to act stronger than human insulin on cell proliferation in selected breast cell lines, [
8,
9] in colon cancer cell lines [
10], and in human lung fibroblasts [
11]
in vitro. Some epidemiological studies seemed to suggest an increased incidence of overall cancer [
12], or breast cancer [
13] in patients treated with an IA, while other clinical studies failed to find an increase in cancer [
14] or the authors concluded that the observed increase in breast cancer reflected allocation bias rather than an effect of insulin glargine [
15]. The best way to overcome these inherent limitations of epidemiological studies would be a well designed, prospective randomized controlled trial. However, the long time required to obtain meaningful resultson the occurrence of cancer in such a trial and the ethical problems of a "carcinogenicity" study in humans [
16] make this approach unfeasible. Therefore, any regulatory decision will be based on a wholistic view integrating epidemiological observational data as well as preclinical studies on possible mechanisms. Overall, data on the effect of insulin glargine on the development of tumours are mostly considered as inconclusive and a change of treatment recommendations or regulatory action seem not justified to experts in the USA [
17] or in Europe [
2,
18,
19].
Whereas the interaction of IAs with the IGF1R is considered the predominant mechanism for the induction of cancer, the European Medicines Agency's "points to consider document" [
4] does not exclude other possible mechanisms:
"Although enhanced Insulin-like Growth Factor 1 (IGF-1) receptor activation and/or aberrant signalling through the insulin receptor have been implicated, the mechanism(s) responsible for the mitogenic activity remain to be clarified." The European Medicines Agency rather emphasize the need for further mechanistic studies:
"The exact mechanism(s) behind the increased carcinogenic and/or mitogenic potential of some insulin analogues remain to be elucidated". Consequently, the non-clinical studies on carcinogenicity of IAs are not to be restricted to IGF1R-mediated effects but "...interaction
with growth factor receptors and mechanisms other than those for IGF-1 need to be taken into account." To minimize the hazard of a carcinogenic or tumour-promoting effect of a new IA, comprehensive experimental investigations of the effects of IAs on normal and malignant cells are recommended. In addition to the standard package of pharmacology, toxicology and safety pharmacology studies, recommendations for some special studies are given for innovative IAs. The receptor binding profile of a new IA should be thoroughly characterized. As the minimum it is recommended to study the effects on insulin receptor and IGF-1R. The ligands human insulin, IGF-1, and AspB10 should be included as controls in all experiments. The quantitative comparison of the mitogenic versus the metabolic activity of an innovative IA is important for the overall conclusion on its biological effects. Always in comparison with human insulin, IGF-1, and AspB10, the metabolic activity should be studied in an assay that has been shown to be most sensitive for effects on the carbohydrate metabolism. In parallel, the mitogenic activity should be studied in an assay that has been shown to be most sensitive for effects on cell proliferation. Finally, the effects on carbohydrate metabolism and cell proliferation should be assessed simultaneously in cells that react to both the mitogenic and the metabolic properties of the IA. It is acceptable, that such cells may be less sensitive to either of the effects.
For a new IA the exact mechanism of carcinogenicity (if any) may be unknown. The IGF1-R mediated increase in cell proliferation may not be the only mechanism by which IAs potentially induce or promote cancer. Therefore, studies in cell culture may not be sufficient. Additional studies in tissues (ex vivo) may be integrated in the standard toxicology studies e.g. in the repeated dose toxicity studies. The selection of strain and species is crucial for the relevance of these studies for humans. It is recommended to use a species or strain for which the mitogenic/metabolic potency ratio is similar to man.
It has been recognized that normal and malignant cells or tissues may react differently to IAs. Additional studies with neoplastic tissues may be helpful and should be given consideration on a case by case basis although this approach clearly exceeds the classical approach in toxicological studies using healthy animals.