R e v i e w s / C o m m e n t a r i e s / P o s i t i o n S t a t e m e n t s
`R E V I E W A R T I C L E
`
`Enhancing Incretin Action for the
`Treatment of Type 2 Diabetes
`
`DANIEL J. DRUCKER, MD
`
`OBJECTIVE — To examine the mechanisms of action, therapeutic potential, and challenges
`inherent in the use of incretin peptides and dipeptidyl peptidase-IV (DPP-IV) inhibitors for the
`treatment of type 2 diabetes.
`
`RESEARCH DESIGN AND METHODS — The scientific literature describing the bio-
`logical importance of incretin peptides and DPP-IV inhibitors in the control of glucose homeosta-
`sis has been reviewed, with an emphasis on mechanisms of action, experimental diabetes, human
`physiological experiments, and short-term clinical studies in normal and diabetic human sub-
`jects.
`
`RESULTS — Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide
`(GIP) exert important effects on ␤-cells to stimulate glucose-dependent insulin secretion. Both
`peptides also regulate ␤-cell proliferation and cytoprotection. GLP-1, but not GIP, inhibits
`gastric emptying, glucagon secretion, and food intake. The glucose-lowering actions of GLP-1,
`but not GIP, are preserved in subjects with type 2 diabetes. However, native GLP-1 is rapidly
`degraded by DPP-IV after parenteral administration; hence, degradation-resistant, long-acting
`GLP-1 receptor (GLP-1R) agonists are preferable agents for the chronic treatment of human
`diabetes. Alternatively, inhibition of DPP-IV–mediated incretin degradation represents a com-
`plementary therapeutic approach, as orally available DPP-IV inhibitors have been shown to
`lower glucose in experimental diabetic models and human subjects with type 2 diabetes.
`
`CONCLUSIONS — GLP-1R agonists and DPP-IV inhibitors have shown promising results
`in clinical trials for the treatment of type 2 diabetes. The need for daily injections of potentially
`immunogenic GLP-1– derived peptides and the potential for unanticipated side effects with
`chronic use of DPP-IV inhibitors will require ongoing scrutiny of the risk-benefit ratio for these
`new therapies as they are evaluated in the clinic.
`
`Diabetes Care 26:2929 –2940, 2003
`
`A fter meal ingestion, nutrient entry
`
`into the stomach and transit
`through the proximal gastrointesti-
`nal (GI) tract stimulates activation of neu-
`ral and hormonal signals that control
`gastric emptying and gut motility, nutri-
`ent absorption, and hormonal regulation
`of energy disposal and storage. The mu-
`
`cosal epithelium of the GI tract is one of
`the earliest integrators of information rel-
`evant to digestion and assimilation of
`nutrient loads. Highly specialized en-
`teroendocrine cells dispersed along the
`length of the GI tract play an important
`role in controlling the rate of gastric emp-
`tying and small bowel motility, pancreatic
`
`● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
`
`From the Banting and Best Diabetes Centre, Department of Medicine, Toronto General Hospital, University
`of Toronto, Ontario, Canada.
`Address correspondence and reprint requests to Daniel J. Drucker, MD, Banting and Best Diabetes Centre,
`University of Toronto, Toronto General Hospital, 200 Elizabeth St., MBRW 4R-402, Toronto, Ontario,
`Canada M5G 2C4. E-mail: d.drucker@utoronto.ca.
`Received for publication 5 May 2003 and accepted in revised form 30 June 2003.
`D.J.D. is on the scientific advisory board for Amylin Pharmaceuticals, Conjuchem, and Transition Ther-
`apeutics and a consultant for Merck, Forest Labs, Bristol Myers Squibb, Triad Pharmaceuticals, Aventis,
`Novartis, Amylin Pharmaceuticals, and Conjuchem.
`Abbreviations: DPP, dipeptidyl peptidase IV; GI, gastrointestinal; GIP, glucose-dependent insulinotropic
`peptide; GIPR, GIP receptor; GLP, glucagon-like peptide; GLP-1R, GLP-1 receptor.
`A table elsewhere in this issue shows conventional and Syste`me International (SI) units and conversion
`factors for many substances.
`© 2003 by the American Diabetes Association.
`
`enzyme secretion, and the growth and
`differentiated absorptive function of the
`small and large bowel epithelium. The
`aim of this review is to examine our cur-
`rent understanding of the physiological
`actions of two gut hormones, glucagon-
`like peptide (GLP)-1 and glucose-
`dependent insulinotropic polypeptide
`(GIP), with an emphasis on the biological
`importance and pharmaceutical potential
`of these peptides for the treatment of type
`2 diabetes.
`
`INTRODUCTION TO THE
`INCRETIN CONCEPT — The devel-
`opment and application of the insulin ra-
`dioimmunoassay to clinical investigation
`has permitted the assessment of ␤-cell se-
`cretory function after meal ingestion in
`normal and diabetic subjects. The obser-
`vation that food ingestion or enteral glu-
`cose administration provoked a greater
`stimulation of insulin release compared
`with similar amounts of energy (glucose)
`infused intravenously (1,2) led to the de-
`velopment of the incretin concept. Hence,
`it was postulated that gut-derived signals
`stimulated by oral nutrient ingestion rep-
`resent potent insulin secretagogues re-
`sponsible for the augmentation of insulin
`release when energy is administered via
`the gut versus the parenteral route (3).
`Although several neurotransmitters and
`gut hormones possess incretin-like activ-
`ity, the considerable evidence from im-
`munoneutralization, antagonist, and
`knockout studies suggests that GIP and
`GLP-1 represent the dominant peptides
`responsible for the majority of nutrient-
`stimulated insulin secretion. The observa-
`tion that patients with type 2 diabetes
`exhibit a significant reduction in the mag-
`nitude of meal-stimulated insulin release
`underlies the interest in determining
`whether defective incretin release or resis-
`tance to incretin action contributes to the
`pathophysiology of ␤-cell dysfunction in
`diabetic subjects.
`
`INCRETIN SYNTHESIS,
`SECRETION, AND
`DEGRADATION — GIP and GLP-1
`are members of the glucagon peptide su-
`perfamily and share considerable amino
`
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`Incretin action in type 2 diabetes
`
`subjects using antisera capable of discrim-
`inating the full-length from the NH2-
`terminally cleaved peptides. The t1/2 of
`infused GIP is ⬃7 and 5 min in normal
`and diabetic human subjects, respectively
`(14). In contrast, the t1/2 of exogenously
`infused intact GLP-1 is considerably
`shorter (13), with intravenously adminis-
`tered GLP-1 eliminated with a half-life of
`⬃2 min in both normal and obese dia-
`betic human subjects (17). Although the
`NH2-terminally truncated peptides
`GIP(3-42) and GLP-1(9-36)amide func-
`tion as weak antagonists of their respec-
`tive receptors (18,19), there is little
`evidence that these truncated peptides ex-
`ert physiologically important actions in
`human subjects in vivo. Despite observa-
`tions that GLP-1(9-36)amide may func-
`tion as an activator of insulin-independent
`glucose clearance in pigs (20), this pep-
`tide does not exert significant glucose-
`lowering properties in human subjects
`(21).
`Circulating levels of GIP(1-42) are
`normal or slightly increased in type 2 di-
`abetic subjects in the basal or postpran-
`dial states (22). In contrast, subjects with
`diabetes or impaired glucose tolerance ex-
`hibit modest but significant reductions in
`levels of meal-stimulated circulating
`GLP-1 (22,23). Furthermore, meal-
`induced increases in GIP and GLP-1 se-
`cretion are inversely correlated with the
`extent of insulin resistance detected in
`human subjects (24). The lower levels of
`circulating GLP-1 detected in diabetic
`subjects are not attributable to altered
`GLP-1 clearance (17). Whether levels of
`meal-stimulated GLP-1 may be restored
`toward normal with improved control of
`diabetes remains unknown.
`
`GIP ACTION: INSIGHTS
`FROM PRECLINICAL AND
`HUMAN STUDIES — GIP was orig-
`inally observed to inhibit gastric acid se-
`cretion (gastric inhibitory polypeptide),
`predominantly at supraphysiological dos-
`ages. Subsequent studies have demon-
`strated potent glucose-dependent insulin
`stimulatory effects from GIP administra-
`tion in dogs and rodents. GIP also regu-
`lates fat metabolism in adipocytes,
`including stimulation of lipoprotein
`lipase activity, fatty acid incorporation,
`and fatty acid synthesis (25). Unlike
`GLP-1, GIP does not inhibit glucagon se-
`cretion or gastric emptying. GIP does pro-
`mote ␤-cell proliferation and cell survival
`
`Figure 1—Structure of preproglucagon and preproGIP encoding GLP-1 and GIP, respectively, is
`shown. The arrow designates the position of the DPP-IV–mediated cleavage after the position 2
`alanine residue. GRPP, glicentin-related pancreatic peptide; IP, intervening peptide.
`
`acid identity. GIP is a single 42⫺amino
`acid peptide encoded within a larger
`153⫺amino acid precursor (Fig. 1) (4).
`GIP-secreting enteroendocrine K-cells are
`concentrated in the duodenum and prox-
`imal jejunum; hence, these cells are ana-
`tomically situated in an ideal location for
`sensing and responding to nutrient inges-
`tion. GLP-1 is derived from a larger pro-
`glucagon precursor that encodes not only
`GLP-1 but also the related proglucagon-
`derived peptides glucagon, GLP-2, oxyn-
`tomodulin, and glicentin (Fig. 1) (5). The
`two forms of GLP-1 secreted after meal
`ingestion, GLP-1(7-37) and GLP-1(7-
`36)amide differ by a single amino acid.
`Both peptides are equipotent and exhibit
`identical plasma half-lives and biological
`activities acting through the same recep-
`tor (6,7); however, the majority (⬃80%)
`of circulating active GLP-1 appears to be
`GLP-1(7-36)amide (8). In contrast to the
`more proximal location of GIP-producing
`K-cells, the majority of GLP-1 is synthe-
`sized within L-cells located predomi-
`nantly in the ileum and colon, although
`GLP-1–producing L-cells have also been
`identified more proximally in the duode-
`num and jejunum. Despite the more dis-
`tal location of most L-cells, circulating
`levels of GLP-1 also increase rapidly
`within minutes of food ingestion. Hence,
`GLP-1 secretion from the distal gut is con-
`trolled by both neural and endocrine sig-
`nals initiated by nutrient entry in the
`proximal GI tract, as well as by subse-
`quent direct contact of open-type L-cells
`with digested nutrients. Ingestion of a
`mixed meal or a meal enriched with spe-
`cific fats and complex carbohydrates is
`
`particularly effective in stimulating GIP
`and GLP-1 release in human subjects
`(9,10). Although the vagal nerve, via M1
`muscarinic receptors, and several neu-
`roendocrine peptides contribute to the
`regulation of GLP-1 release in rodents
`(11,12), the factors responsible for rapid
`nutrient-stimulated GLP-1 release in hu-
`man subjects are largely unknown.
`The levels of total circulating GIP and
`GLP-1 immunoreactivity reflect a combi-
`nation of intact, full-length active and
`NH2-terminally truncated inactive pep-
`tides, with GIP(3-42) and GLP-1(9-
`36)amide contributing to ⬎50% of total
`immunoreactive GIP and GLP-1 in both
`the fasting and the postprandial states
`(13,14). Plasma levels of both GIP and
`GLP-1 immunoreactivity are low in the
`fasting state and rise rapidly within min-
`utes of food ingestion. Initial studies of
`circulating levels of GIP and GLP-1 relied
`principally on radioimmunoassays inca-
`pable of distinguishing the biologically
`active full-length peptides from inactive
`COOH-terminal peptide fragments gen-
`erated as a result of proteolytic cleavage.
`Studies have demonstrated that both GIP
`and GLP-1 were cleaved at the position 2
`alanine by the widely expressed amino-
`peptidase dipeptidyl peptidase IV (DPP-
`IV) (15,16). These findings have
`prompted a reanalysis of the circulating
`molecular forms of GIP and GLP-1 using
`newer radioimmunoassays more specific
`for the full-length bioactive peptides in
`normal and diabetic subjects.
`The disappearance of exogenously
`administered GIP and GLP-1 has been
`studied in normal and diabetic human
`
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`Drucker
`
`in islet cell line studies (26,27); whether
`GIP also induces ␤-cell growth or survival
`in diabetic rodents remains unclear.
`The physiological actions of GIP have
`been deduced using GIP peptide antago-
`nists, GIP receptor antisera, and GIP re-
`ceptor knockout mice. NH2-terminally
`truncated or modified GIP peptides such
`as GIP(6-30)amide, GIP(7-30)amide, or
`(Pro3)GIP block GIP binding to the GIP
`receptor with varying effectiveness, and
`attenuate the insulinotropic effects of ex-
`ogenous GIP in vitro and endogenous GIP
`in vivo (28 –30). Similarly, immunopuri-
`fied antisera against the extracellular do-
`main of the GIP receptor block GIP
`binding and attenuate glucose-dependent
`insulin secretion after oral glucose load-
`ing in rats and mice (31,32). Complemen-
`tary evidence for the incretin-like actions
`of GIP is derived from analysis of GIP
`receptor⫺null mice, which exhibit mild
`glucose intolerance after oral glucose
`loading (33). Surprisingly, GIPR⫺/⫺ mice
`exhibit resistance to diet-induced obesity
`after months of high-fat feeding. More-
`over, the GIPR⫺/⫺ genotype attenuates
`obesity in the ob/ob mouse, possibly be-
`cause of reduced fat storage and altered
`lipid metabolism as a direct result of ab-
`sent GIP receptor (GIPR) action in adipo-
`cytes (34). Whether GIPR action
`significantly modulates adipocyte biol-
`ogy, lipoprotein synthesis, and weight ac-
`cretion in humans is not known.
`In contrast to the potent glucose-
`lowering actions of GIP in normal ro-
`dents, exogenous GIP administration is
`comparatively less insulinotropic in obese
`diabetic rodents. GIP levels are increased
`in some models of experimental rodent
`diabetes, and continuous GIP infusion for
`4 h produces GIPR desensitization in nor-
`mal rats (35). ZDF rats exhibit normal
`levels of GIP, absent insulinotropic re-
`sponses to exogenous GIP and reduced
`expression of the GIPR in isolated islets
`(36). Recent studies with more potent GIP
`analogs engineered for resistance to
`DPP-IV have demonstrated improved in-
`sulinotropic and glucose-lowering prop-
`erties after peptide administration to both
`normal and diabetic rodents (37–39).
`Infusion of porcine or human GIP
`into patients with type 2 diabetes has pro-
`duced variable insulinotropic responses,
`ranging from preserved (40) to attenuated
`or near absent insulin secretion (41– 45).
`The potential for ␤-cell GIP responsivity
`to improve with treatment in type 2 dia-
`
`Figure 2—The major biological actions of GLP-1.
`
`betic subjects is intriguing, but has not
`been extensively examined (46). The GIP
`defect in insulin secretion seems most
`pronounced in the late phase of insulin
`secretion (47). Moreover, ⬃50% of nor-
`moglycemic first-degree relatives of type
`2 diabetic subjects exhibit reduced insu-
`lin secretion after exogenous GIP infusion
`(48). Hence the reduced insulinotropic
`action of GIP in diabetes likely reflects a
`combination of genetic and acquired de-
`fects. Whether the pancreatic effects of
`GIP on ␤-cell proliferation and survival
`are also diminished in experimental or
`clinical diabetes is not known.
`
`GLP-1 PRECLINICAL
`STUDIES AND
`PHYSIOLOGICAL ACTIONS —
`Original observations elucidating a role
`for GLP-1 in the potentiation of glucose-
`dependent insulin secretion (49 –51) and
`insulin gene expression (52) were fol-
`lowed by experiments demonstrating that
`GLP-1 also inhibits glucagon secretion
`(53,54) and gastric emptying (55) (Fig.
`2). Acute intracerebroventricular (56) in-
`jection of GLP-1 or GLP-1 receptor (GLP-
`1R) agonists produces transient reduction
`in food intake, whereas more prolonged
`intracerebroventricular or peripheral
`GLP-1R agonist administration is associ-
`ated with weight loss in some (57– 60),
`but not all (61) studies. GLP-1 actions on
`food intake appear related in part to over-
`lapping actions on central nervous system
`aversive signaling pathways, which re-
`mains a topic of intense interest (62– 66).
`In contrast to GIP, the spectrum of actions
`
`delineated for GLP-1 that promote glu-
`cose lowering (regulation of insulin and
`glucagon secretion, inhibition of gastric
`emptying, and reduction of food intake)
`appear comparable in diabetic versus
`nondiabetic animals of various ages.
`GLP-1 exerts actions on ␤-cells inde-
`pendent of acute stimulation of insulin se-
`cretion. Incubation of isolated rat islet
`cells with GLP-1 recruited nonresponsive
`glucose-resistant ␤-cells to a functional
`state of glucose-responsive insulin secre-
`tion, designated glucose competence
`(67,68). GLP-1R agonists also promote
`insulin biosynthesis, ␤-cell proliferation,
`and survival (69 –71), and stimulate dif-
`ferentiation of exocrine cells or islet pre-
`cursors toward a more differentiated
`␤-cell phenotype (72–74). The GLP-1R–
`dependent augmentation of ␤-cell mass
`has been demonstrated in diverse experi-
`mental models, including neonatal rats
`administered streptozotocin and ex-
`endin-4 (75) and normal Wistar rats ages
`6 and 22 months infused with native
`GLP-1 for 5 days (76). Similarly, GLP-1R
`agonists promote ␤-cell proliferation and
`expansion of functional islet mass after
`partial pancreatectomy in rats aged 4 –5
`weeks (69) or in neonatal rat pups sub-
`jected to experimental
`intrauterine
`growth retardation (77). The expansion
`of ␤-cell mass after GLP-1R agonist ad-
`ministration prevents or delays the occur-
`rence of diabetes in db/db mice (78) and
`GK diabetes-prone rats (79). Further-
`more, the induction of islet proliferation
`after GLP-1R activation has been seen
`with a broad range of GLP-1R agonists,
`
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`Incretin action in type 2 diabetes
`
`including native GLP-1 (76,79,80), ex-
`endin-4, NN2211 (81), and CJC-1131
`(82).
`GLP-1R agonists also activate anti-
`apoptotic pathways coupled to a reduc-
`tion in ␤-cell death. db/db mice treated
`with exendin-4 for 2 weeks exhibited de-
`creased numbers of apoptotic ␤-cells, re-
`duced pancreatic caspase-3 activation,
`and increased Akt1 expression (78). Re-
`duced islet apoptosis has been observed
`in GLP-1–treated Zucker diabetic rats
`(83) and in exendin-4 –treated mice after
`streptozotocin-induced ␤-cell injury
`(70). The anti-apoptotic actions of
`GLP-1R agonists are likely direct, as
`GLP-1 reduced peroxide-induced apo-
`ptosis in Min6 insulinoma cells (84) and
`exendin-4 significantly attenuated cyto-
`kine-induced apoptosis in cultures of pu-
`rified rat ␤-cells (70). Hence, the GLP-
`1R– dependent activation of both
`proliferative and anti-apoptotic pathways
`in the pancreas provides complementary
`mechanisms for preserving and enhanc-
`ing functional ␤-cell mass.
`The physiological
`importance of
`GLP-1 action has been studied using
`GLP-1R antagonists. Infusion of the pep-
`tide exendin(9-39) into rats, mice, ba-
`boons, and humans produces an increase
`in fasting glucose and glycemic excursion
`after oral glucose loading in association
`with reduced levels of circulating insulin
`(32,85– 87). Exendin(9-39) also pro-
`duces abnormal glycemic excursion after
`nonenteral glucose loading in mice (32).
`These findings illustrate that transient
`disruption of GLP-1 action consistently
`perturbs the incretin and nonincretin ac-
`tions of GLP-1 on glucoregulation. Acute
`intracerebroventricular injection of ex-
`endin(9-39) increases food intake in sati-
`ated rats (56), whereas repeated daily
`intracerebroventricular administration of
`exendin(9-39) increases food intake and
`weight gain (57). Similarly, acute ex-
`endin(9-39) administration increases gas-
`tric emptying after glucose ingestion in
`fistulized rats (88). Comparable studies
`with exendin(9-39) in humans have dem-
`onstrated the essential role of GLP-1 ac-
`tion for glucose control via regulation of
`glucagon and insulin secretion (89,90).
`Hence, the majority of actions observed
`after exogenous administration of GLP-
`1R agonists are also physiologically essen-
`tial, as revealed by acute interruption of
`GLP-1 action.
`Genetic disruption of GLP-1R expres-
`
`sion in mice has produced comparable in-
`sights into the physiological importance
`of GLP-1 action. GLP-1R⫺/⫺ mice exhibit
`abnormal glucose tolerance after both
`oral and intraperitoneal glucose challenge
`in association with diminished glucose-
`stimulated insulin secretion. In contrast,
`insulin sensitivity and the glucagon re-
`sponse to glucose loading or hypoglyce-
`mia are normal in the absence of GLP-1R
`signaling (91). Consistent with the car-
`diovascular effects of GLP-1 in rodents,
`GLP-1R⫺/⫺ mice exhibit defective cardio-
`vascular responses to stress (92). Despite
`the potential importance of GLP-1R cir-
`cuits for transducing the anorectic action
`of leptin (93), GLP-1R⫺/⫺ mice retain
`normal to enhanced leptin sensitivity
`(94,95). Similarly, food intake and body
`weight are not significantly perturbed in
`GLP-1R⫺/⫺ mice in the CD1 genetic
`background (96,97). In contrast, GLP-
`1R⫺/⫺ mice manifest subtle but detect-
`able abnormalities in islet number and
`size (98) and exhibit a defective ␤-cell re-
`generative response to partial pancreatec-
`tomy (99). Hence, GLP-1R actions are
`physiologically important for the growth
`and adaptive regeneration of murine
`␤-cells.
`
`GLP-1R agonists and experimental
`models of diabetes
`The glucose-lowering action of GLP-1 de-
`lineated in nondiabetic animals has been
`demonstrated in multiple models of ex-
`perimental diabetes. A 48-h infusion of
`native GLP-1 lowered blood glucose in
`association with increased levels of circu-
`lating insulin, islet insulin content, and
`insulin mRNA in Wistar rats aged 22
`months (100), and perfused pancreas
`studies have demonstrated GLP-1–
`dependent augmentation of insulin secre-
`tion in ZDF rats of diverse genetic
`backgrounds (101–103). Similarly, ex-
`endin-4 lowered glucose in db/db and
`ob/ob mice, ZDF rats, and diabetic rhesus
`monkeys in acute and chronic experi-
`ments (78,104,105) and the GLP-1 ana-
`log NN211 improved glycemic control in
`pigs, rats, and mice (60,81,106). Remark-
`ably, glucose tolerance remained signifi-
`cantly improved in ZDF rats for weeks
`after a 48-h infusion of native GLP-1
`(107). This “memory effect” for sustained
`improvement of glycemic control was
`also observed in db/db mice after discon-
`tinuation of therapy with CJC-1131, an
`albumin-bound GLP-1R agonist (82).
`
`GLP-1 action in human subjects
`The majority of GLP-1 actions delineated
`in preclinical experiments have also been
`demonstrated in human studies. Infusion
`of GLP-1(7-36)amide into normal human
`subjects stimulated insulin secretion, re-
`duced glucagon secretion, and signifi-
`cantly reduced blood glucose in the
`fasting state after glucose loading or meal
`ingestion (6,50,108). In contrast to GIP,
`the insulinotropic and glucose-lowering
`actions of GLP-1 are preserved in human
`subjects with type 2 diabetes (45,109) in
`both the fasting and the postprandial
`states (7). Similarly, GLP-1 inhibits gas-
`tric acid secretion (110,111) and gastric
`emptying in humans (55) and the GLP-1–
`dependent attenuation of gastric empty-
`ing contributes to decreased glycemic
`excursion and, consequently, reduced
`glucose-stimulated insulin secretion
`(112,113). Consistent with the impor-
`tance of gastric emptying and glucagon
`secretion for glycemic control, GLP-1 also
`lowers blood glucose in type 1 diabetic
`subjects (114 –116). Analogous to studies
`demonstrating the induction of glucose
`competence in rodent ␤-cells, GLP-1 in-
`fusion enhances ␤-cell function and insu-
`lin secretory dynamics in human subjects
`with impaired glucose tolerance or type 2
`diabetes (117–119). GLP-1 may also en-
`hance glucose clearance in humans
`(120,121); however, the majority of these
`actions are likely mediated indirectly
`through effects on insulin and glucagon
`(122–124). Although several reports have
`described the effects of GLP-1 on muscle,
`liver, and fat cells, experimental evidence
`demonstrating expression of the GLP-1
`receptor in these tissues in vivo is lacking.
`Hence, the indirect actions of GLP-1,
`leading to improvement in glycemic con-
`trol and reduction in free fatty acids, may
`explain observations of improved insulin
`sensitivity in GLP-1–treated diabetic sub-
`jects (125).
`The effect of GLP-1 in restoring glu-
`cose competence in rodent islets has
`prompted studies of GLP-1 action and
`␤-cell function in type 2 diabetic patients.
`Insulin–treated diabetic subjects previ-
`ously classified as “sulfonylurea non-
`responders” exhibited ␤-cell GLP-1
`responsivity, with lowering of fasting and
`postprandial glucose in association with
`enhanced insulin secretion (126). Pa-
`tients treated with both GLP-1 and glib-
`enclamide exhibited a greater degree of
`glucose reduction compared with the ef-
`
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`DR. REDDY’S LABORATORIES, INC.
`IPR2024-00009
`Ex. 1023, p. 4 of 12
`
`

`

`fect of either agent alone (127). Similarly,
`the combination of GLP-1 and metformin
`was shown in a 48-h crossover study to be
`more effective for lowering blood glucose
`than monotherapy with either agent alone
`(128).
`The GLP-1– dependent suppression
`of glucagon secretion raises the possibility
`that GLP-1 therapy will be associated with
`an increased risk of hypoglycemia and po-
`tentially defective counterregulation if
`glucagon secretion remains suppressed in
`the face of GLP-1–linked hypoglycemia.
`Rapid gastric emptying may be associated
`with enhanced GLP-1 release and an in-
`creased risk of hypoglycemia in postgas-
`trectomy patients (129). Similarly, acute
`administration of GLP-1 (80 nmol) to
`nondiabetic subjects in the fasted state
`produced mild relative hypoglycemia in
`some subjects (mean glucose ⬃3.5
`mmol/l) (130). Nevertheless, appropriate
`glucagon responses to hypoglycemia do
`not appear to be blunted in GLP-1–
`treated subjects (130), and GLP-1 in-
`fusion does not impair normal counter-
`regulatory responses to hypoglycemia in
`healthy human subjects (131). Hence, the
`risk of hypoglycemia seems modest in
`type 2 diabetic subjects treated with
`GLP-1R agonists alone.
`The demonstration that both intrace-
`rebroventricular and peripheral adminis-
`tration of GLP-1R agonists induces
`weight loss in preclinical experiments has
`fostered interest in the potential actions of
`these agents to diminish appetite and re-
`duce weight gain in overweight human
`subjects. The majority of human studies
`have examined appetite and food inges-
`tion over short (24-h) time periods after
`single-dose injection or continuous infu-
`sion of GLP-1. Small but statistically sig-
`nificant reductions in appetite and meal
`ingestion have been recorded in studies of
`normal, obese, and diabetic GLP-1–
`treated subjects (132–136). A meta-
`analysis of available data from 115
`subjects demonstrated significant GLP-1–
`dependent reductions in energy con-
`sumption in lean and overweight subjects
`(137). The acute reduction in food con-
`sumption and inhibition of gastric emp-
`tying has been detected even with
`physiological increases in levels of circu-
`lating GLP-1 (136). Administration of
`GLP-1 via continuous subcutaneous infu-
`sion for 6 weeks to obese diabetic subjects
`was associated with reduced appetite and
`a small but significant mean 1.9-kg
`
`weight loss (125). Hence, GLP-1 therapy
`in human subjects appears associated
`with prevention of weight gain or modest
`weight loss; however, long-term data are
`not yet available.
`Although single or repeated subcuta-
`neous injections of native GLP-1 decrease
`blood glucose in human subjects
`(138,139), the glucose-lowering effects
`are transient and no longer evident 1–2 h
`after peptide injection (140,141). Fur-
`thermore, continuous enhancement of
`GLP-1 action for 24 h/day appears supe-
`rior for glucose control compared with
`peptide infusion for 16 h (142). Contin-
`uous intravenous or subcutaneous infu-
`sion of GLP-1 in short- and long-term
`studies has been shown to be highly effec-
`tive in lowering blood glucose in diabetic
`subjects (125,143,144), but this intensive
`and expensive approach has major limita-
`tions for the treatment of large numbers of
`diabetic patients. The rapid degradation
`and clearance of native endogenous and
`exogenously administered GLP-1 (145)
`have spurred the clinical development of
`degradation-resistant GLP-1 analogs with
`longer durations of action in vivo.
`Exendin-4 is a naturally occurring
`39⫺amino acid GLP-1R agonist isolated
`from the salivary gland venom of the liz-
`ard Heloderma suspectum (146). Ex-
`endin-4 exhibits 53% amino acid identity
`to mammalian GLP-1 (146,147), yet
`binds to and activates the GLP-1 receptor.
`Furthermore, exendin-4 is highly resis-
`tant to the proteolytic activity of DPP-IV
`and exhibits a longer duration of action in
`vivo. Intravenous infusion of exendin-4
`lowered fasting and postprandial blood
`glucose in normal healthy volunteers and
`was associated with a 19% reduction in
`calorie consumption assessed during a
`single test meal (148). Exendin-4 exerted
`similar effects on insulin secretion after
`acute intravenous infusion in diabetic
`subjects (149), and subcutaneous daily
`administration of exendin-4 to subjects
`with type 2 diabetes significantly reduced
`blood glucose and HbA1c (a decline from
`9.1 to 8.3%) over a 1-month treatment
`period (150). Exendin-4 has been evalu-
`ated in eight phase 2 trials in 323 individ-
`uals with type 2 diabetes who received
`dosages of 0.05–2.0 ␮g/kg subcutane-
`ously. Nausea and vomiting were the
`principal side effects observed (151). A
`4-week treatment period produced a sig-
`nificant reduction in HbA1c levels, with
`sustained reduction in postprandial gly-
`
`Drucker
`
`cemia maintained over the 28-day treat-
`ment period.
`Exendin-4 treatment (0.08 ␮g/kg
`s.c., b.i.d. or t.i.d.) over 1 month was
`evaluated in 109 patients treated with sul-
`fonylureas or metformin, alone or in com-
`bination. The treatment was generally
`well tolerated, with three subjects with-
`drawing in the first 12 days because of
`nausea. At the end of the study period, a
`significant reduction was observed in lev-
`els of serum fructosamine, HbA1c, and
`mean postprandial glucose, but no signif-
`icant change was noted in body weight or
`serum lipids (152). Antibodies against ex-
`endin-4 were detected in 19% of treated
`subjects; however, the antibodies did not
`affect treatment responses. In all, 15% of
`patients experienced hypoglycemia; all of
`these subjects received sulfonylureas plus
`exendin-4 (152). Exendin-4, recently re-
`named exenatide, is currently being eval-
`uated for the treatment of type 2 diabetes
`in phase 3 trials in combination with met-
`formin, sulfonylurea agents, or both.
`NN2211 (liraglutide) is a fatty acid–
`linked DPP-IV–resistant derivative of
`GLP-1 designed for subcutaneous admin-
`istration that exhibits a pharmacokinetic
`profile compatible with once-daily injec-
`tion (153). NN2211 reduced fasting and
`postprandial glycemia in diabetic subjects
`after a single 10 ␮g/kg subcutaneous in-
`jection at 11:00 P.M., in association with
`inhibition of gastric emptying and re-
`duced levels of circulating glucagon (154).
`NN2211 has been tested in phase 2 clin-
`ical trials. Additional approaches for pro-
`longing the duration of action of GLP-1
`derivatives include the use of albumin-
`bound GLP-1 molecules (82) and sus-
`tained release exendin-4 preparations;
`however, human data with these pharma-
`ceutical approaches is currently limited.
`
`Inhibition of DPP-IV for the
`treatment of type 2 diabetes
`The observation that GLP-1 and GIP are
`rapidly cleaved at the position 2 alanine
`leading to inactivation of their biological
`activity (15,16) has fostered interest in the
`development of inhibitors of DPP-IV, the
`principal enzyme responsible for incretin
`inactivation (155,156). DPP-IV is but one
`member of a large family of related en-
`zymes with overlapping enzyme specific-
`ity; however, adenosine deaminase
`affinity chromatography that specifically
`binds DPP-IV removes 95% of DPP-IV–
`
`DIABETES CARE, VOLUME 26, NUMBER 10, OCTOBER 2003
`
`2933
`
`MPI EXHIBIT 1023 PAGE 5
`
`DR. REDDY’S LABORATORIES, INC.
`IPR2024-00009
`Ex. 1023, p. 5 of 12
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`

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`Incretin action in type 2 diabetes
`
`Table 1—Properties and biological actions of GIP and GLP-1
`
`GIP
`
`GLP-1
`
`42–Amino acid peptide
`Released from duodenum
`NH2-terminal inactivation by DPP-IV
`Stimulates insulin secretion
`Minimal effect on gastric emptying
`No effect on glucagon secretion
`No regulation of satiety and body weight
`Promotes expansion of ␤-cell mass
`Normal GIP secretion in diabetic subjects
`Defective GIP response in type 2 diabetes
`
`30/31–Amino acid peptide
`Released from distal small bowel and colon
`NH2-terminal inactivation by DPP-IV
`Stimulates insulin secretion
`Inhibits gastric emptying
`Inhibits glucagon secretion
`Inhibits food intake and weight gain
`Promotes expansion of ␤-cell mass
`Reduced GLP-1 secretion in dia