Glargine and Cancer: CanWeNowSuggest Closure?

The two publications on the pharma-cokinetics of insulin glargine in in-dividuals with type 1 and type 2diabetes in this issue ofDiabetes Care (1,2)provide additional explanatory evidencein support of the denitive ndings in theOutcome Reduction with Initial GlargineIntervention (ORIGIN) study in whichexposure to insulin glargine for a medianduration of 6.2 years did not increase therisk of any cancer (hazard ratio 1.00 [95%CI 0.881.13]) or death from cancer(0.94 [0.771.15]) (3).

Soon after glargines long awaited andwelcome introduction into clinical prac-tice in 2000, questions were raised aboutits safety prole. This conception wasbased on the early nding that insulinglargine had an enhanced afnity toIGF-1 receptors when tested in a humanosteosarcoma cell line (Saos/B10) with apreponderance of IGF-1 receptors and as-sociated with increased mitogenicity(proliferation in an existing tumor cellline) (4). These ndings were akin tothose observed with the AspB10 insulinanalog, the development of which hadearlier been discontinued because of anincrease in both benign and malignantmammary gland tumors in Sprague-Dawley rats after 12 months exposure(5,6); thus AspB10 was referred to as thecarcinogenic insulin analog (7). In con-trast, detailed extensive toxicological life-time carcinogenicity studies of insulinglargine in animals (rodent and nonro-dent), albeit at lower doses, revealed nocarcinogenicity signal with insulinglargine when compared with human in-sulin (8,9). The risk was regarded as smallby the European Medicines Agency safetyworking party because of the lack of effectonmammary gland proliferation, absenceof mammary carcinoma, and rare tumorsduring the lifetime studies in animals. Thehigher insulin receptor afnity and con-sequent prolonged dephosphorylationdue to the increased residence time ofAspB10 on insulin receptors (4) alongwith a predilection for the insulin receptorIR-A isoform (10)may in large part accountfor the differential metabolic andmitogenicoutcomes observed when compared withhuman insulin and insulin glargine.

Di-arginyl insulin (ArgB31-ArgB32hu-man insulin) is known to be an intermedi-ate in the conversion of proinsulin toinsulin. Insulin glargine (GlyA21-ArgB31-ArgB32 human insulin) possesses glycineat A21 instead of asparagines, therebyadjusting its solubility at neutral pH forretarded release while lowering its pro-pensity for aggregation. After subcutaneousadministration, insulin glargine precipi-tates amorphously and is then slowlyreleased according to zero-order kinetics.It has been documented for some time thatinsulin glargine is biotransformed both inthe subcutaneous tissue and circulation(11). Sequential cleavage of the carboxylterminus of the C-chain occurs via localand systemic converting proteases intothe primary metabolite GlyA21 human in-sulin (M1) and also GlyA21 des-ThrB30human insulin (M2) as described usinghigh-performance liquid chromatographyand a nonspecic radioimmunoassay. Bio-transformation in serum occurs rapidlywith ;70% of insulin glargine convertedtoM1within 30min of incubation (12,13).The metabolites M1 and M2 retain the fullbiological activity of human insulin andhave substantially reduced IGF-1 receptorbinding and mitogenic potency relative tohuman insulin (4). Whereas insulin glar-gine has a greater afnity to IGF-1 receptorsthan human insulin (10-fold) it has a 100-fold lower afnity than native IGF-1 (14).This enhanced IGF-1 afnity is reversed byremoval of the di-arginyl molecules. Theprimary (M1) and secondary (M2) metab-olites are less metabolically active with re-duced afnity to IGF-1 receptors andequivalent growth-promoting activity inSaos-2 and MCF-7 cell lines comparedwith human insulin (15).

The early vital information on the bio-transformation of insulin glargine (11) wasseemingly ignored for many years, with re-search groups having chosen instead to fo-cus on the in vitro IGF-1 binding propertiesof the parent compound insulin glargine.Therefore, little further development hasoccurred until the current articles by Bolliand colleagues (1,2). In the meantime, re-searchers have extensively explored the po-tential adverse clinical impact of insulinglargine ondiabetic retinopathy and cancer.

Concerns relating to the developmentor progression of diabetic retinopathy byinsulin glargine were alleviated when inboth a meta-analysis of four phase 3 trials(16) and a randomized control trial over a5-year period revealed no difference be-tween NPH and insulin glargine (17). InSeptember of 2009, four internationalcommunications relating to insulin andthe risk of cancer were published together(1821); three were instigated by the Euro-pean Association for the Study of Diabetesto explore the validity of the rst studycarried out in Germany (18), which de-monstrated a strong correlation betweeninsulin dose and cancer risk and impliedthat insulin glargine carried a higher riskthan human insulin. This lead to consider-able anxiety among the insulin-treated di-abetic population and the diabetes carecommunity alike, which necessitated theregulatory authorities and diabetic associa-tions to issue statements of reassurance.The ndings of Hemkens et al. (18) weresubsequently criticized because they usedan unconventional and fundamentallyawed analysis that adjusted for insulindosage, which meant that the conclusionswere unsupportable (22). There was noexcess cancer risk seen in a 5-year random-ized control trial comparing NPH and in-sulin glargine (17) or when the combinedrandomized control trial experience of ma-lignancies in studies using insulin glarginewere evaluated (23). With regard to thegeneral clinical question, the ConsensusReport on Cancer and Diabetes by theAmerican Diabetes Association and Amer-ican Cancer Society concluded that fur-ther research was needed to clarify therelationship between exogenous insulinand increased cancer risk and the specicquestion relating to insulin glargine (24).Sandow (9) attempted to highlight thecomplexity of the relationship betweenthe growth effects of insulin and insulinanalogs while emphasizing the need forclarity regarding the meaning of mitogenic-ity under physiological and pathophysio-logical situations in relationship to insulinand insulin analogs.

In hindsight, it is somewhat unfortu-nate that during the discussion of insulinglargine, after its introduction into clinical

2426 DIABETES CARE, VOLUME 35, DECEMBER 2012 care.diabetesjournals.org

C O M M E N T A R Y ( S E E B O L L I E T A L . , P . 2 6 2 6 , A N D L U C I D I E T A L . , P . 2 6 4 7)

practice, the published results of the toxi-cological studies that demonstrated nobiological (mitogenic) signal in the lifetimestudies in animals (at maximum tolerateddoses), which could be attributed to theslightly enhanced IGF-1 receptor afnity,received little attention. The concept of thedi-arginyl insulins mimicking nature withthe retarding principle residing in themolecule itself (solubility) in contrast tomolecules requiring protamine, excess zincor, more recently, acylated fatty acids wasalso seemingly forgotten. It is only rela-tively recently that this latter aspect hasbeen revised after the in vitro studies,which conrm the reduced IGF-1 receptorbinding and low mitogenic potency of theactive metabolites (M1 and M2) of insulinglargine. Therefore, the delay in fullyexploring the pharmacokinetics of insulinglargine in humans after the early studieswith limited methodologies by Kuerzelet al. (11) and Agin et al. (12) has beenmost unfortunate. Because of technicalconstraints, a detailed description of thepharmacokinetics of insulin glargine andits metabolites (M1 and M2) in humanshas hitherto not been possible. This wasdue to cross-reactivity between insulinglargine and its primary metabolite in theradioimmunoassays then used (25).

The long-awaited need for morecomprehensive pharmacokinetic investi-gations of insulin glargine in humans isaddressed by the two accompanyingarticles by Bolli and colleagues (1,2).From a clinical point of view, it is essentialto provide information about the circulat-ing concentrations of the parent com-pound and its active metabolites usingadequate methodologies. These two stud-ies present the rst such data in individ-uals with both type 1 (1) and type 2 (2)diabetes using a newly developedspecic assay method involving liquidchromatography-tandem mass spec-trometry capable of providing discretemeasurements of insulin glargine andits metabolites M1 and M2 after theirextraction from human plasma usingimmunoafnity columns. Insulin glarginewas administered to individuals with type1 diabetes by bolus subcutaneous injectionat both therapeutic and supratherapeuticdoses (0.3, 0.6, and 1.2 units/kg) in aeuglycemic clamp used to dene theglucodynamic changes over the study periodof 30 h. Each participant received a singledose of insulin glargine. The parent com-pound, insulin glargine, and the metaboliteM2 were rarely detected in plasma abovethe lower limit of detection at 33 pmol/L

(;6 mU/mL) regardless of the dose (up to1.2 units/kg). M1 was detected in plasmain a dose-related fashion correlating withthe observed glucodynamic changes ob-served. Importantly, insulin glargine wasnot detected inplasma at the higher dosage.Essentially similar ndingswere seen in thetype 2 study after the administration of in-sulin glargine at only a single dose of 0.4units/kg, with themetaboliteM1being pre-dominant in the plasma with little or noparent insulin glargine or the second me-tabolite M2 detectable by involving liquidchromatography-tandem mass spectrome-try (2). A similar plasma insulin prole wasobserved when the radioimmunoassay wasused, indirectly conrming the major con-tribution by the M1 metabolite. There isacknowledgment by the authors that it isalso necessary and advisable to examine themetabolism of insulin glargine after pro-longed exposure and at even higher doses,which are sometimes used in obese andinsulin-resistant individuals.

Therefore, the virtual absence of theparent compound insulin glargine in thecirculation after its subcutaneous injec-tion invalidates the submission that the invitro ndings of enhanced IGF-1 bindingand mitogenicity of insulin glargine has aclinical correlate, especially as insulinglargine is extensively and quickly me-tabolized in the subcutaneous tissue andin the systemic circulation to itsmetabolitesM1 and M2, both of which have lessermetabolic and similar mitogenic potency tohuman insulin (15,26).

The important and denitive ndingsby Bolli and colleagues (1,2) represent acritical piece of evidence in support of therecent ndings of the ORIGIN trial (3)and the French National Healthcare In-surance Database (27), neither of whichobserved an excess risk of cancer duringlong-term exposure to insulin glargine. Itis, however, surprising that it has taken al-most 12 years since insulin glargine wasrst introduced to start to truly understandits metabolic fate in humans after subcuta-neous administration despite the early ob-servations by Kuerzel et al. (11).

In response to concerns raised by thelaboratory ndings of Kurtzhals et al. (4)and the awed interpretation of the epi-demiological study triggered by a Germanhealth insurance initiative based on reim-bursement considerations (18), a largevolume of clinical and experimental datahave been generated during this interven-ing, diversionary period. Research ad-dressing the question whether diabetesand/or diabetes therapy inuences the

risk of cancer has since provided exten-sive and invaluable information on thecomplex interrelationship between diabe-tes, its treatment, and the various types ofcancers. However, on the basis of thendings from epidemiological studies(3,27) and the latest welcome and over-due pharmacokinetic data (1,2)whichis further supported by similar ndingsin young children (28)the chapter onwhether insulin glargine per se is an in-dependent risk factor for cancer shouldnow be closed.

DAVID R. OWENS, CBE, MD, FRCP

From the Institute of Molecular and ExperimentalMedicine, Cardiff University School of Medicine,University Hospital of Wales, Cardiff, U.K.

Corresponding author: David R. Owens, owensdr@cardiff.ac.uk.

DOI: 10.2337/dc12-1968 2012 by the American Diabetes Association.

Readers may use this article as long as the work isproperly cited, the use is educational and not forprot, and the work is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ fordetails.

AcknowledgmentsD.R.O. has received lec-ture fees and honoraria for serving on advisoryboards of sano-aventis, Roche, and Boeh-ringer Ingelheim. No other potential conictsof interest relevant to this article werereported.

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