Introduction
Clinical trials in patients with type 1 and type 2 diabetes mellitus have established that maintenance of near-normoglycaemic control reduces the risk of microvascular complications, such as diabetic retinopathy [
1‐
5]. To facilitate reaching glycaemic goals without excessive hypoglycaemia, analogues of human insulin with improved pharmacokinetic and pharmacodynamic action profiles have been developed [
6‐
9].
Insulin glargine (A21Gly,B31Arg,B32Arg human insulin) is a recombinant, long-acting basal insulin analogue that differs from human insulin by the substitution of glycine for asparagine at position 21 of the A chain and the addition of two arginines at the C-terminus of the B chain. These changes produce an insulin with a duration of action of approximately 24 h or more after subcutaneous injection, without pronounced peaks of activity [
10]. In clinical trials, a single daily injection of insulin glargine provides glycaemic control equivalent to that afforded by neutral protamine Hagedorn (NPH) insulin [
11‐
15], but with a lower risk of hypoglycaemia [
16,
17].
One of the five 6 month, phase 3 registration trials of insulin glargine [
14] reported that the percentage of participants with an increase of three or more steps on the Early Treatment Diabetic Retinopathy Study (ETDRS) retinopathy severity scale was greater in the insulin glargine group (7.0%) than in the human NPH insulin group (2.7%). This finding may have been related to chance or to differences at baseline in the patients studied, especially since no retinopathy entrance criteria were specified and the data showed more of a ‘scatter’ and did not follow a natural progression [
18]. However, because of the potential importance of this finding, a carefully designed clinical trial of sufficient duration and statistical power was conducted to address the long-term safety of insulin glargine compared with human NPH insulin. Here we report the findings of this trial, which used change on the ETDRS scale based on the masked, centralised grading of seven-field stereoscopic fundus photographs as the primary measure of retinopathy progression in patients with type 2 diabetes mellitus [
19].
Discussion
This study was specifically designed to detect differences in the incidence of retinopathy progression by fundus photography over a 5 year period. To ensure the greatest possible objectivity of this assessment, retinopathy scoring was conducted at a single ophthalmologic centre, in a standardised fashion, by specially trained and certified graders who were masked to treatment. Throughout the study, changes in ETDRS scores in each group were remarkably similar, with no significant differences between treatment groups. Likewise, there were no significant differences in the number of patients with clinically significant macular oedema and proliferative diabetic retinopathy development between the insulin glargine and NPH insulin groups. Collectively, these results demonstrate that treatment of type 2 diabetes mellitus with insulin glargine over 5 years was not associated with an increase in progression of diabetic retinopathy, compared with NPH insulin treatment.
Because the primary objective of this study was to examine effects on diabetic retinopathy, and not to specifically evaluate intensive glycaemic lowering, similar levels of glycaemic control were sought by the treatment strategy, to minimise any confounding effect on the interpretation of the retinal findings. Despite this intent, a small difference in HbA1c levels in favour of twice-daily NPH insulin (7.6% vs 7.8%; p = 0.0053) was observed between the two treatments at endpoint. The statistical significance was probably detected because of the large sample size required for the retinopathy primary analysis. Furthermore, the upper bound of the 95% CI for HbA1c treatment effect (0.35%) was lower than the non-inferiority margin of 0.4% commonly used to compare two glucose-lowering treatments. Given that control of FPG was similar between the two groups, the lower HbA1c levels in the NPH insulin arm of the study were presumably due to lower glucose levels during the day, likely related to the higher doses of the twice-daily NPH insulin regimen compared with once-daily insulin glargine, and perhaps to the higher percentage of patients combining prandial insulin with NPH insulin. Of note, in the exploratory analysis on the cohort of approximately one third of the study population who took basal insulin only, mean HbA1c levels at any visit were nearly identical in both groups for most of the study. In any event, the presence of lower HbA1c levels of 0.1% at baseline and 0.2% at study end in the NPH insulin arm strengthens the observation that, if different at all, the retinal changes were slightly less with insulin glargine than with NPH insulin treatment. Lending further support to this interpretation is the somewhat greater prevalence of retinopathy in the insulin glargine group at baseline, both in terms of medical history and of more severe ETDRS scores, which might predispose these patients to faster progression of diabetic retinopathy.
This is the longest controlled study ever reported using insulin glargine. It has demonstrated sustained glycaemic improvements and a reduced risk of severe hypoglycaemia compared with NPH insulin. Furthermore, the average weight gain was approximately 0.74 and 0.96 kg/year for insulin glargine and NPH insulin, respectively, over the 5 year study.
The lack of increase in progression of retinopathy seen with long-term insulin glargine treatment in this study is a point of discussion in terms of the hypothesis that the greater binding affinity of insulin glargine compared with NPH insulin for the IGF-1 receptor may lead to accelerated progression of retinopathy (a marker of mitogenicity), as suggested by some in vitro studies [
23,
24]. It should be noted that the majority of in vitro and in vivo studies have shown that insulin glargine is not associated with increased mitogenic activity at physiological concentrations [
25‐
28], and that mitogenicity is only seen in studies using cancer cell lines and when insulin is administered at supraphysiological concentrations [
24,
26]. There is further evidence against problematic glargine-induced signalling at the insulin receptor: insulin glargine dissociates rapidly from the receptor, in contrast to the mitogenic B10Asp insulin, which dissociates at a much slower rate [
29].
The frequency of retinopathy progression observed in this study was similar to that previously reported in the UK Prospective Diabetes Study (UKPDS), a large, long-term, randomised trial of intensive treatment in patients with newly diagnosed type 2 diabetes mellitus [
2,
4]. In a subset of UKPDS patients with adequate fundus photographs at both the baseline and 6 year visits (all treatment groups combined), progression of retinopathy by two or more steps on the ETDRS scale was observed in 467 of 1,919 patients (24.3%), which is very similar to the 5 year two or more step progression observed in our patients (approximately 25%). Caution is required when comparing these frequencies because of differences between the studies, in particular the side-by-side comparison of baseline and follow-up photographs used by the Wisconsin grading centre to confirm progressions of three or more steps, which may have reduced the frequency observed in our study. The recently diagnosed type 2 diabetes patients in the UKPDS and the patients in the present study (mean diabetes duration 10 years) also differed in terms of the presence of retinopathy at baseline (defined as at least one microaneurysm in at least one eye; 37% vs 61%, respectively).
In summary, this long-term, adequately powered study robustly confirms that treatment with insulin glargine does not confer any greater risk of worsening of diabetic retinopathy than does NPH insulin treatment [
18]. Moreover, it provides further new information regarding the long-term safety of insulin glargine compared with NPH insulin, beyond that of ophthalmic safety, demonstrating that similarly sustained improvements in glycaemic control with a lower risk of hypoglycaemia can be obtained with once-daily insulin glargine compared with twice-daily NPH insulin.
Acknowledgements
This study was sponsored by sanofi-aventis. Editorial support was provided by N. Gingles through the global publications group of sanofi-aventis. We would like to thank all investigators and patients who participated in this study. Biostatistical analysis, based on the entire raw data set and evaluation of the study protocol and prespecified plan for data analysis, was provided by X. Guo (sanofi-aventis) and were independently confirmed by C. L. Yau of Tulane University. Data from this manuscript were presented as an oral presentation at the American Diabetes Association 2008 congress (San Francisco, CA, USA, 6–10 June 2008).
Duality of interest
J. Rosenstock has received grants for research from and/or has been a consultant to Amylin, Boehringer-Ingelheim, Bristol-Myers Squibb, Centocor, Eli Lilly, Emisphere, GlaxoSmithKline, Johnson & Johnson, MannKind, Merck, Novartis, Novo Nordisk, Pfizer, Roche, Sankyo, sanofi-aventis and Takeda. V. Fonseca has received research support (to Tulane University) with grants from GlaxoSmithKline, Novartis, Novo Nordisk, Takeda, Astra-Zeneca, Pfizer, sanofi-aventis, Eli Lilly, Daiichi-Sankyo, Novartis, the National Institutes of Health (NIH) and the American Diabetes Association (ADA), and honoraria for consulting and lectures from GlaxoSmithKline, Novartis, Takeda, Pfizer, sanofi-aventis and Eli Lilly. J. McGill has received grant support (to Washington University) from sanofi-aventis, Pfizer, Eli Lilly, Novo Nordisk, Novartis, GlaxoSmithKline, Elixir, Tolerx, Biodel, Mannkind, Takeda; served on advisory boards and speakers' bureaus for sanofi-aventis and Novo Nordisk, and on speakers' bureaus for Eli Lilly, Merck, Novartis, Novo Nordisk, sanofi-aventis, Daiichi-Sankyo, Forest and GlaxoSmithKline; and has been a consultant/advisor for Merck, Novo Nordisk, Elixir, Mannkind, Amgen. I. Hramiak has received research grant support from Pfizer, sanofi-aventis, Novo Nordisk and Eli Lilly and has served on advisory boards for GlaxoSmithKline, Novo Nordisk, sanofi-aventis and Merck. J.-P. Hallé has received grants for research from and/or has been a consultant and/or on the speaker bureau of Bristol-Myers Squibb, ConjuChem, Bellus Health, Eli Lilly, GlaxoSmithKline, Merck Frosst, Novartis, Novo Nordisk, Pfizer, Roche, sanofi-aventis and Takeda. M. Riddle has received grants for research and/or honoraria for consulting or lectures from Amylin, Lilly, the Amylin-Lilly Alliance, Novo Nordisk, Pfizer, sanofi-aventis and Valeritas. P. Johnston is an employee of, and holds stock options from, sanofi-aventis. M. Davis has received research support from sanofi-aventis.