2.1. Critical structural elements of GLP-1 sequence

Early structure-activity studies of endogenous GLP-1 revealed the key sequence domains that correlate with potency and enzymatic degradation. Studies have suggested that the N-terminal residues His7, Gly10, Phe12, Thr13 and Asp15 are essential for interaction with the GLP-1 receptor, as substituting these residues with L-Ala leads to a severe loss of affinity for its receptor [43]. Similarly, C-terminal positions Phe28 and Ile29 are also important for receptor binding due to their role in maintaining the peptide’s secondary structure [43]. Endogenous GLP-1 has a short plasma half-life because of its rapid degradation by DPP-4, which cleaves GLP-1 between the Ala8 and Glu9 amino acids [44]. In addition, neutral endopeptidase (NEP) plays a minor role in GLP-1 degradation by cleaving at six sites, Asp15-Val16, Ser18-Tyr19, Tyr19-Leu20, Glu27-Phe28, Phe28-Ile29 and Trp31-Leu32, within the central and C-terminal peptide domains [44].

Based on structure-activity studies, many GLP-1 analogs have been designed to evade enzymatic degradation while maintaining their potency. Because DPP-4 cleavage at Ala8-Glu9 is the major reason for the rapid deactivation of native GLP-1, replacing Ala8 is a commonly used strategy to reduce DPP-4 degradation. For example, replacing Ala8 with a-aminoisobutyric acid (Aib) was found to completely prevent DPP-4 degradation [45]. Similarly, in two other commercialized GLP-1 analogs, albiglutide and dulaglutide, Ala8 is substituted with Gly to prevent DPP-4 degradation (Figure 1).

2.2. Exendin-4 and its analogs

In addition to human GLP-1 analogs, a naturally-occurring peptide and a hormone found in the saliva of Gila monster (a venomous lizard), exendin-4, was identified as a GLP-1 RA (Figure 1). Although exendin-4 shares only 53% homology with human GLP-1, in vitro studies demonstrated that exendin-4 has a slightly higher potency than native GLP-1 [46], attributable to their high similarity of key N-terminal residues. Compared to native GLP-1, exendin-4 is more resistant to cleavage by DPP-4 due to a Gly to Ala substitution at the second position [47]. Furthermore, the Leu29-Ser39 segment of exendin-4 can form a tertiary fold, or “Trp cage”, which conformationally shields residues 21-39, protecting these sites from being cleaved by NEP 24.11 [48, 49]. The enzyme resistance results in a distinct metabolic pathway of exendin-4. Studies reported that exendin-4 is mainly degraded on kidney membranes and eliminated via glomerular filtration [50, 51]. In comparison, native GLP-1 is eliminated by both the kidneys and liver while also subject to metabolism in peripheral tissues.

Exenatide, a synthetic version of exendin-4, was the first GLP-1 RA to be approved by the FDA under the brand name of Byetta® (Table 1). In 2009, Byetta® was approved as a stand-alone therapy for type 2 diabetes by the FDA [19]. Byetta® is supplied in multi-dose prefilled pens consisting of 250 μg/ml synthetic exenatide solution in pH 4.5 acetate buffer with mannitol (tonicity-adjusting agent) and metacresol (preservative). It can be stored at 2-8°C for 3 years before use, and stored at room temperature below 25 °C for 30 days after first use (Table 2) [19]. Byetta® is administered as a twice daily (BID) subcutaneous injection before each main meal (Table 1). Patients begin with a 5 μg dose for one month, followed by titration up to 10 μg [19]. Pharmacokinetic parameters of exenatide are summarized in Table 3. Peak plasma concentration is achieved about 2 hours post administration. The product half-life is 2.4 hours, which allows for twice-daily administration, and is a large improvement over endogenous GLP-1 [19]. Animal studies have shown that exenatide is mainly cleared by renal filtration.

Another successful GLP-1 RA, lixisenatide, was launched in the U.S. in July 2016 under the brand name Adlyxin® (Table 1). Modified from exendin-4, lixisenatide is a 44-amino-acid peptide with the C-terminal Pro replaced by 6 Lys residues (Figure 1). In vitro studies found that lixisenatide has four times greater affinity for the GLP-1 receptor than native GLP-1 and exenatide [52]. Adlyxin® is administered once daily at 10 – 20 μg per day (Table 1). The pharmacokinetic profile of lixisenatide is similar to that of exenatide with a characteristic kidney elimination and Tmax of 2 hours post administration. The apparent volume of distribution is 100 L, with approximately 55% of lixisenatide bound to plasma proteins in circulation (Table 3) [25]. The half-life of lixisenatide is 2.6 hours, which is similar to that of exenatide [25]; however, unlike exenatide, lixisenatide is indicated as a once-daily injection. This is partly due to lixisenatide’s better ability to slow gastric emptying, yielding a better glucose control and, thus, reduced administration frequency [53]. Similar to Byetta®, Adlyxin® is supplied in multi-dose prefilled pens. The formulation of Adlyxin® contains lixisenatide 50 or 100 μg/ml in pH 4.5 acetate buffer, with glycerol 85% (cosolvent), methionine (antioxidant) and metacresol (preservative). Adlyxin® can be stored at 2-8°C for 2 years before use, and stored at room temperature below 30 °C for 14 days after first use (Table 2) [25, 31].

Although amino acid substitution is an effective strategy to reduce proteolytic degradation, modified GLP-1 analogs are still subject to rapid renal clearance, making it difficult to further prolong peptide circulation time. Thus, other drug delivery strategies were used to develop longer-acting GLP-1 RAs, as described below.

2.3. Fatty acid conjugates

Covalent attachment of a fatty acid, known as acylation, is used extensively to control the half-life of therapeutic peptides [54]. One example is the insulin detemir (Levemir®), in which myristic acid is introduced to insulin at position B29. The conjugation of myristic acid results half-life extension from 4-6 min (insulin) to 5-7 hours (insulin detemir).

Acylation with fatty acids can increase the circulation time of therapeutic peptides by several mechanisms. First, acylated peptides could self-assemble to supramolecular structures due to their amphiphilic nature. Once subcutaneously injected, acylated peptide spontaneously forms a depo at the injection site, leading to a delayed absorption into systemic circulation [55]. In the case of acylated GLP-1 RAs, it was reported that liraglutide, a palmityolated GLP-1 RA, could self-assemble into hexa-, hepta- or octamers, depending on the pH and ionic strength [56-58]. When the acylated peptide monomer finally dissociates from the oligomer and enters blood circulation, it readily yet reversibly binds to serum albumin through hydrophobic and ionic interactions between the fatty acids and albumin [59, 60]. Albumin binding provides steric hindrance against enzymatic degradation of the acylated peptide and slows its renal clearance due to the large size of the peptide-albumin complex. The albumin-bound peptide is also hypothesized to interact with the neonatal Fc receptor (FcRn), allowing it to shuttle through the albumin recycling pathway and evade intracellular degradation, thus, significantly increasing the circulation half-life [61]. However, although the FcRn recycling is proposed as a half-life extension mechanism for albumin-bound peptides in general, to the best of our knowledge there is no existing experimental evidence that proves FcRn recycling occurs for acylated GLP-1 RAs.

The stability and potency of acylated GLP-1 RAs depends on the site of acylation, the bulkiness and length of the fatty acid chains, and the spacer used between the peptide and fatty acid [62-64], Because the N-terminus is key for receptor binding and activation, acylation in the N-terminal region leads to serious loss of potency. In contrast, C-terminal modifications have little impact on potency [63]. The spacers linking peptide segment and fatty acid chains also greatly affect the potency of acylated GLP-1 RAs [64]. While the bulkiness of fatty acid was found to negatively impact the potency of GLP-1 RAs, the length of the fatty acid had more complex effects [64]. The length of fatty acid chain is positively correlated with the binding affinity to albumin, leading to a longer circulation time [62, 64]. However, the peptide must dissociate from albumin to activate GLP-1 receptor, thus GLP-1 RAs with longer fatty acid chain exhibit lower potency in the presence of serum [63, 64]. Therefore, the acylation site and the length of fatty acid chain must be selected to achieve the right balance between potency and in vivo stability.

The most successful example of an acylated GLP-1 is liraglutide, which was approved by the FDA in 2010 under the brand name Victoza®. Liraglutide, derived from native GLP-1, has a 16-C fatty acid linked to Lys26 with a γ-Glu spacer and substituting Arg for Lys at position 34 (Figure 1, ​,2)2) [65]. Once administered, the fatty acid chain causes oligomerization of liraglutide, enabling formation of oligomers at the injection site [56, 58]. The self-association of liraglutide leads to delayed absorption into systemic circulation, which was observed in clinical studies where intravenously administered liraglutide was found to have a shorter residence time than when administered subcutaneously [66, 67]. In plasma, liraglutide monomers dissociate from the oligomers [67] and bind to albumin within the central blood compartment [68]. In vitro studies showed that while liraglutide could be metabolized by DPP-4 and NEP in a manner similar to GLP-1, its degradation was much slower [69], possibly due to the steric hindrance provided by fatty-acid-bound albumin. Albumin binding also prevents renal filtration of liraglutide, evidenced by the fact that no intact liraglutide could be detected in patients’ urine in clinical studies [69]. Combined, these factors all contribute to enhance the in vivo stability of liraglutide. Pharmacokinetic data showed that liraglutide has a longer half-life, about 9-13 hours [20], compared to the other short-acting GLP-1 Ras, such as exenatide (2.1 hours) [19]. This affords liraglutide the ease of once-daily injections. Pharmacokinetic analysis also suggests that, compared to exenatide BID, liraglutide has smaller peak-to-trough fluctuations and better continuous exposure [66]. Liraglutide (Victoza®) is approved in the U.S. as a second-line type 2 diabetes treatment (Table 1). Victoza® is supplied in multi-dose prefilled pens. The formulation of Victoza® contains liraglutide 6 mg/ml in phosphate buffer with propylene glycol (cosolvent) and phenol (preservative). With a pKa of about 9.5, the solubility of liraglutide decreases below pH 7, requiring the formulation’s pH to be adjusted to 8.15 [70]. Victoza® can be stored at 2-8°C for 30 months before use. After first use, the product can be stored at room temperature at 2-8°C or 15-30°C for 30 days (Table 2) [20, 33]. Patients initiate the treatment with a 0.6 mg/dose for one week then increase to a 1.2 mg/dose. The dose can be increased to a 1.8 mg/dose for additional glycemic control (Table 1). In addition, a higher dose liraglutide (3 mg/dose) has also been approved by the FDA for weight management under the brand name Saxenda® [16].

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Figure 2.

(A) Half-life extension mechanisms of acylated GLP-1 analogs. (B) Mean concentration-time profile of liraglutide following single dose of 0.6 mg, 1.2 mg, or 1.8 mg at steady state in healthy male Chinese subjects. Adapted with permission of © John Wiley & Sons, Inc. from Jiang et al. J Clin Pharmacol. 2011;51:1620–7. [72] (C) Mean concentration-time profile of semaglutide following single dose of 1.0 mg at steady-state. Adapted with permission of © John Wiley & Sons, Inc. from Kapitza et al. J Clin Pharmacol. 2015; 55(5):497-504. [73]

Semaglutide—which was approved by the FDA in December 2017 under the brand name of Ozempic®—is a next generation GLP-1 RA based on liraglutide. Compared to liraglutide, semaglutide has an additional amino acid substitution in the 8th position from Ala to α-aminobutyric acid (Aib) to avoid degradation by DPP-4 [64]. A longer stearic (C18) di-acid fatty chain was introduced into semaglutide, whereas liraglutide has a C16 fatty acid [64]. The spacer between the stearic di-acid chain and peptide backbone has been optimized [62], and the spacers composed of γGlu or 8-amino-3,6-dioxaoctanoic acid (ADO) were found to be optimal for the potency of GLP-1 RAs, though mechanisms are not clear. For semaglutide, the fatty di-acid chain is attached to Lys26 through one glutamic acid and two 8-amino-3,6-dioxaoctanoic acid (ADO) moieties to achieve the optimal binding affinity [64]. Semaglutide shows a higher albumin binding affinity in comparison to liraglutide, which may be a result of the longer fatty acid side chain included in semaglutide [64]. The GLP-1 receptor binding affinity of semaglutide is slightly lower than that of liraglutide, which can be attributed to the longer lipid chain and presence of the terminal acid group on steric acid [64]. In clinical studies, semaglutide was shown to be degraded by enzymes prior to renal extraction. Because of its slow enzyme degradation and reduced renal extraction, semaglutide has a half-life of 7 days, allowing for once-weekly administration (Figure 2, Table 3) [71]. Despite the reduced dosing frequency, the dose of semaglutide — initially 0.25 mg/dose with the ability to increase to 1.0 mg/dose—is lower than that of liraglutide, which starts with a 0.6 mg/dose (Table 1) [26]. Similar to Victoza®, Ozempic® is supplied in multi-dose prefilled pens containing semaglutide 0.75 mg/ml in phosphate buffer (pH 7.4) with propylene glycol (cosolvent) and phenol (preservative). Ozempic® can be stored at 2-8°C for 3 years prior to use. After first use, the product can be stored at room temperature at 2-8°C or 15-30°C for 56 days. (Table 2) [26, 35].

2.4. Albumin fusion

Albumin is well-recognized as a potential carrier of therapeutic peptides due to its long circulation half-life, ability to distribute broadly in the body and low immunogenicity. This strategy has been successfully used in the development of the FDA-approved drug Idelvion® (albumin fusion of coagulation factor IX) [74]. Albumin conjugation likely improves GLP-1 RA plasma half-life by several mechanisms, including providing steric hindrance from proteolysis and increasing molecular weight to reduce kidney filtration (Figure 3). In addition, covalent attachment to albumin may also prolong the peptide’s circulation time by a unique albumin recycling pathway through the neonatal Fc receptor (FcRn) [75]. Following endocytosis by endothelial cells, albumin binds to the FcRn receptor within the acidic lysosomal environment, which enables albumin rescue from degradation, transport back to the cell’s surface and return to the circulation [75, 76]. Such a recycling mechanism is believed to contribute to the prolonged half-life of endogenous albumin and, presumably, albumin-fusion proteins [77].

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Figure 3.

(A) Half-life extension mechanisms of albiglutide. (B) Mean concentration-time profile of albiglutide following a single subcutaneous 30 mg injection. Adapted with permission of © 2015 Taylor & Francis Ltd. from Young et al. Postgraduate Medicine. 126:7, 84-97. www.tandfonline.com [78].

An albumin conjugation strategy was used to design albiglutide (Tanzeum®), which was approved by the FDA in 2014. Albiglutide is a recombinant protein composed of two copies of GLP-1 analogs fused to human albumin. The molecule has a Gly8 to Ala substitution in both copies of the GLP-1 analogs to improve resistance to DPP-4 degradation. Albiglutide is administered once weekly by subcutaneous injection and is thought to be absorbed through the lymphatic circulation [23]. The peak plasma concentration is achieved about 4 days after a single administration, and steady-state is obtained after the first 3-4 weekly doses (Table 3) [23]. Due to its high molecular weight, renal filtration is not expected to be an important elimination mechanism for albiglutide. Overall, albiglutide has an in vivo half-life of 6-8 days, and is approved for once-weekly administration. In Tanzeum®, albiglutide is lyophilized with mannitol (stabilizer/tonicity adjuster), trehalose dihydrate (stabilizer/tonicity adjuster), polysorbate 80 (surfactant), and sodium phosphate (pH adjuster). The lyophilized cake and solvent (water for injection) are supplied in single-dose dual-chamber pens (Table 2) [23, 36]. Reconstitution is needed prior to use to make the albiglutide solution of 60 mg/ml or 100 mg/ml for injection. The product can be stored 3 years at 2 – 8 °C or 4 weeks at room temperature below 25 °C [23, 36]. Patients are initiated at 30 mg/dose (equivalent to 0.82 μmol GLP-1 RA per dose) and can be increased to 50 mg/dose (1.37 μmol/dose) if necessary (Table 1) [23].

2.5. Fc fusion

Fc fusion is another established strategy to prolong the half-life of therapeutic peptides. Several Fc fusion products, such as etanercept (TNFR-Fc fusion protein, Enbrel®) and aflibercept (VEGFR-Fc fusion, Zaltrap®), are approved by the FDA [79]. Overall, incorporating the Fc domain of IgG prolongs a peptide’s circulation time through several mechanisms. First, the Fc domain can endow hybrid proteins with a longer in vivo circulation time through its interactions with FcRn. Similar to albumin’s recycling process, FcRn can bind with the Fc domain in an acidic endosomal environment to protect the Fc domain from degradation, while disassociating from the Fc domain in neutral pH environment outside the cell [80]. The important role played by FcRn in prolonging a peptide half-life was evidenced in Suzuki et al. [81], where the affinities of Fc-fusion proteins for FcRn were closely correlated to their half-lives. Fc fusion also reduces renal filtration of peptides by increasing their molecular weight, which is especially favorable for GLP-1 RAs that are eliminated primarily by the kidney [19]. Fc fusion also reduces DPP-4 degradation of GLP-1, likely due to steric hindrance by the bulky IgG fragment [82]. From a technological viewpoint, Fc-fusion to GLP-1 allows for an easier purification process relative to albumin fusion due to widely available protein A binding columns used for IgG purification [83].

An important concern associated with the development of GLP-1 Fc fusion products is their potential safety risk arising from the ability of the Fc fragment to trigger antibody-dependent cell-mediated (ADCC) cytotoxicity and complement-mediated cell lysis through Fc-γ receptor interactions. When compared to the IgG1 Fc domain, the IgG2 and IgG4 Fc domains may be more favorable in the development of GLP-1 RAs due to their lower affinity for Fc-γ and complement receptors [84, 85]. The potency of GLP-1 RA after Fc fusion is another concern, as linking the GLP-1 analog with the IgG1 hinge region directly results in a severe loss of potency, suggesting a suitable spacer is needed for potency retention [86].

The most successful GLP-1 product employing the Fc-fusion strategy is dulaglutide (Trulicity®), approved by the FDA in 2014 as an adjunctive therapy for type 2 diabetes. Dulaglutide is a disulfide-bonded homodimer fusion peptide with each monomer consisting of one GLP-1 analog moiety and one IgG4 Fc region (Figure. 1 and ​and4).4). Within the GLP-1 analog, Ala8 is replaced by Gly to resist DPP-4 degradation, and Gly36 is substituted for Arg to avoid a potential T-cell epitope interaction [86]. The IgG4 Fc region was selected and its sequence modified at several positions to further reduce its interaction with the Fc-y receptors to avoid ADCC immunologic side effects [86]. A spacer comprising 16 amino acids was added between the GLP-1 analog and the Fc moiety to prevent activity loss. Dulaglutide has an extended half-life of 4.7 days, allowing for once weekly dosing. Trulicity® is supplied in single-dose pens or syringes. The formulation consists of dulaglutide solution at a concentration of 1.5 mg/ml or 3 mg/ml in citrate buffer with mannitol (stabilizer/tonicity adjuster) and polysorbate 80 (surfactant) [37]. The product can be stored at 2 – 8 °C for 2 years or below 30 °C for 14 days (Table 2) [24, 37]. Patients begin therapy with a 0.75 mg/dose once weekly (corresponding to 0.024 μmol GLP-1 RA/dose), and the dose can be doubled for additional glycemic control (Table 1) [23].

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Figure 4.

(A) Half-life extension mechanisms of dulaglutide. (B) FcRn recycling pathway of dulaglutide. (C) Mean concentration-time profile of dulaglutide following single dose of 1.5 mg subcutaneously injected in abdomen, thigh, or arm. Data obtained from an EMA assessment report of Trulicity® [87].

2.6. Sustained-release drug delivery systems

Another strategy to achieve sustained plasma levels of GLP-1 RAs is by taking advantage of drug delivery systems. Various systems including polymeric hydrogels [88], nanoparticles [89] and microparticles [90] have been tested in preclinical studies. However, only poly(lactic-co-glycolic acid) (PLGA) based implants of GLP-1 RA have been approved by the FDA thus far. PLGA is well accepted by regulatory authorities as a safe and efficacious way to deliver therapeutic peptides [91, 92]. Lupron Depot®, a microsphere formulation of leuprolide acetate and Zoladex®, a millicylinder formulation of goserelin, are examples of FDA approved PLGA-based sustained release products [93, 94]. In these products, peptides are loaded within a PLGA polymer which slowly degrades in the body, allowing for sustained release of the therapeutic peptide. For example, Lupron Depot® can release the loaded leuprolide over an extended period of 1 or 6 months, depending on the specific polymer formulation used [95]. Such an extended release profile enables maintenance of target peptide concentrations with fewer administrations, thus improving patient compliance.

The PLGA delivery approach was applied in the design of a polymer microsphere formulation of exenatide in collaboration between Eli Lilly, Amylin and Alkermes [96]. Although the degradation of PLGAs can be tuned from a few weeks to more than six months, exenatide PLGA microspheres fall short of a more desirable monthly administration, and the final product, Exenatide QW (Bydureon®), was launched as a once-a-week product [96]. Once injected subcutaneously, the 60-μm-diameter microspheres hydrate and adhere to each other, forming a matrix drug reservoir in situ. A discontinuous drug release profile with a low initial burst and two peaks of drug release at the 2nd and 7th week is observed, corresponding to different mechanisms of exenatide release from microspheres (Figure 5A and ​andB)B) [79]. During the first 48-hour “burst release” stage exenatide is released from the surface and surface pores of the microspheres. This product was specifically designed to have low initial drug burst in order to avoid nausea and vomiting side effects [96]. The second stage is characterized by the slow diffusion of exenatide out of the polymer matrix with the maximum diffusion rate and corresponding plasma concentration peak achieved at about 2 weeks. The third stage of drug release results from the hydrolysis and erosion of the PLGA polymer [96]. The maximum drug-release rate at this stage is achieved at 7 weeks, which is evidenced by the second peak of the blood concentration-time curve. The overall release of exenatide can last for 11 weeks [21]. After the first 6-7 weekly administrations of Bydureon®, a stable steady state plasma level of exenatide is achieved at 300 pg/ml as peaks and valleys from weekly dosages at different injection sites begin to overlap (Figure 5C) [97]. From the formulation perspective, Bydureon® is supplied as single-dose trays and single-dose dual-chamber pens where the exenatide PLGA microparticle powders and diluent are supplied separately. Prior to use, patients need to resuspend the PLGA microparticles containing 2 mg exenatide in 0.65 ml phosphate buffer, which also contains sodium chloride (tonicity adjuster), carboxymethylcellulose sodium (suspending agent) and polysorbate 20 (surfactant) (Table 2) [21]. To achieve better usability, a new formulation, Bydureon® Boise™, was developed and approved by the FDA in 2017. In Bydureon® Boise™, the PLGA powders are suspended in medium chain triglycerides supplied in single-dose autoinjectors, which avoids the cumbersome procedures before use and enhances the patient experience [39]. Despite the different formulations, Bydureon® and Bydureon® Boise™ are both administered subcutaneously with 2 mg/dose and have similar pharmacokinetics profiles (Table 1) [21, 39]. Bydureon® can be stored at 2 – 8 °C for 3 years or at the room temperature below 25 °C for 4 weeks (Table 2) [21, 38].

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Figure 5.

(A) Release process of exenatide from microspheres. (B) Mean concentration-time profile of exenatide following single dose of Bydureon®. (C) Mean concentration-time profile of exenatide following repeated weekly administrations of Bydureon® (exenatide ER same as exenatide QW) compared with a single administration of Byetta® (exenatide IR). Adapted with permission of © 2011 Adis Data Information BV from Fineman et al. Clin Pharmacokinet 2011; 50 (1).[97]

As discussed above, one of the drawbacks of PLGA microparticles in the Bydureon® formulation is that they cannot deliver GLP-1 RAs at a steady rate. To address this problem and achieve steady and continuous delivery of GLP-1 RAs, other controlled-release devices such as osmotic pumps have been developed. One representative product is ITCA 650, developed by Intarcia Therapeutics. ITCA 650 is a subcutaneously implanted matchstick-sized osmotic mini-pump, which continuously releases exenatide for 6 months in a zero-order manner, making twice-yearly administration possible [98]. The zero-order release of exenatide results in a favorable pharmacokinetic profile of ITCA 650 for several reasons. Pharmacokinetic studies showed the ITCA 650 could achieve steady-state exenatide concentration within 5 days following implantation, compared to 6-7 weeks for exenatide QW [42]. The plasma concentration of exenatide remained steady over the period of treatment [42]. Furthermore, the drug treatment can be discontinued if necessary by removal of the implant. In the clinical trials, once the implanted pump was removed, exenatide levels became undetectable by 24 hours [42]. A New Drug Application for ITCA 650 was submitted to the FDA in November 2016 [99]. If approved, it would be the first needle-free GLP-1 RA on the market.

3.1. Glycemic control patterns

GLP-1 RAs can efficiently regulate plasma glucose levels through multiple mechanisms, including stimulation of insulin production, inhibition of glucagon secretion and slowing gastric emptying. Though both short-acting and long-acting GLP-1 RAs show favorable glucose control efficacy, they have different effects on fasting plasma glucose (FPG) and postprandial plasma glucose (PPG) levels. Clinical studies have demonstrated that short-acting GLP-1 RAs have a greater efficacy in lowering PPG level, while long-acting GLP-1 RAs predominantly affect FPG levels. For example, in a head-to-head clinical trial comparing exenatide BID (Byetta®) and exenatide QW (Bydureon®), patients treated with exenatide BID had a lower PPG, while patients treated with exenatide QW had a lower FPG [106]. As exenatide was the same active ingredient in both groups, this data suggests that PK profile significantly influences pharmacological activity of short-acting versus long-acting GLP-1 RAs, resulting in differential reduction of PPG and FPG levels.

It is believed that a product’s effect on FPG level is directly related to pharmacokinetics of GLP-1 RA and GLP-1 receptor activation pattern, which, in turn, leads to the differential secretion of insulin and glucagon during a fasting state [107]. Short-acting GLP-1 RAs usually have limited drug exposure within the body due to rapid elimination. Exenatide, for example, administered one hour before the morning and evening meal, is almost completely eliminated from the plasma after 10 hours [108]. Thus, the GLP-1 plasma concentration in the middle of the day and overnight is low, leading to modest efficacy on FPG regulation. On the contrary, long-acting GLP-1 RAs that are present in the circulation between injections enables sustained GLP-1 receptor activation and continuous stimulation of insulin secretion. In the clinical trial comparing exenatide BID (Byetta®) and liraglutide (Victoza®), the fasting insulin level of the Victoza®-treated group increased on average by 12.43 pmol/L, while that of Byetta® decreased by 1.38 pmol/L [109]. Increased insulin secretion during a fasting state was also observed for other long-acting GLP-1 RAs such as dulaglutide (Trulicity®) [110] and albiglutide (Tanzeum®) [111].

The efficacy of GLP-1 RAs on PPG, however, is thought to primarily result from delayed gastric emptying rather than stimulation of insulin production [112]. An inverse relationship between gastric emptying rate and PPG has been demonstrated in clinical trials [113, 114]. Short-acting GLP-1 RAs such as exenatide BID (Byetta®) and lixisenatide (Adlyxin®) can significantly delay gastric emptying processes in vivo. The delayed gastric emptying leads to the reduced plasma glucose excursion. As a result, postprandial insulin concentrations are actually dose-dependently decreased, by exenatide and lixisenatide despite the known ability of GLP-1 RAs to stimulate insulin production [113, 114]. On the other hand, long-acting GLP-1 RAs have only a modest effect on gastric emptying, possibly caused by tachyphylaxis due to continuous activation of the GLP-1 receptor [115, 116]. For example, in a clinical study comparing exenatide BID (Byetta®) and exenatide QW (Bydureon®), it was found that after 14 weeks of administration, Byetta® still induced significant retardation of gastric emptying, while Bydureon® had little effect on gastric mobility [106]. Consequently, long-acting GLP-1 RAs typically lead to less profound reductions of PPG compared to short-acting GLP-1 RAs [112].

3.2. Impact of various GLP-1 RAs on glycated hemoglobin (HbA1c) levels

Glycated hemoglobin (HbA1c) is a biomarker extensively used by diabetologists to evaluate the ability of patients to control overall blood sugar level over periods of weeks to months. The higher the HbA1c level, the worse a patient’s glycemic control and the higher the likelihood that that individual will develop diabetes-related complications [117]. All commercialized GLP-1 RAs have proved to effectively reduce levels of HbA1c (Table 4). Though directly comparing the HbA1c reduction of different GLP-1 RAs is challenging, several head-to-head clinical studies have provided insight into differences between these products based on the pharmacodynamic and efficacy results from clinical trials [118]. Compared with short-acting products, long-acting GLP-1 RAs usually present better efficacy in reducing HbA1c, which may result from their ability to better control FPG levels. The DURATION-1 and DURATION-5 clinical trials, which directly compared exenatide BID (Byetta®) to exenatide QW (Bydureon®), reported better HbA1c level reduction for Bydureon® [119, 120]. Another clinical trial, LEAD-6, showed greater reduction of HbA1c levels in patients treated with long-acting Victoza® relative to short-acting Byetta® (−1.12% versus −0.79%, p < 0.0001) [109]. Similarly, dulaglutide (Trulicity®) reduced HbA1c levels more effectively than exenatide twice daily (Byetta®) in the AWARD-1 trial (−1.51% for dulaglutide 1.5 mg, −1.30% for dulaglutide 0.75 mg versus −0.99% for exenatide twice daily, p < 0.001 for both dulaglutide groups relative to exenatide) [121].

Differences in efficacy among GLP-1 RAs belonging to the same class seem less significant. No statistically significant differences were found between the short-acting compounds Byetta® and Adlyxin® in the head-to-head GetGoal-X trial [53]. Among long-acting GLP-1 RAs, the HbA1c-reduction efficacy of Victoza® (liraglutide) was not significantly different from that of Trulicity® (dulaglutide) [121] or Tanzeum® (albiglutide) [122]. The DURATION-6 trial demonstrated that Victoza® showed greater reduction of HbA1c levels than Bydureon® [123]. However, additional retrospective meta-analyses of various clinical data sets found no meaningful difference in HbA1c reduction between Victoza® and Bydureon® [124-126].

3.3. Weight loss

GLP-1 receptor activation may lead to significant weight loss by suppressing appetite, thus reducing calorie intake [127]. Though delayed gastric emptying and GI side effects of GLP-1 RAs are tempting explanations for the weight loss, their correlation with weight loss in diabetic patients was contradicted by several clinical trials [128, 129]. Instead, the anorectic effect of GLP-1 RAs was found to be responsible for the weight loss, which is mainly attributed to the satiating effect or rapid hunger satisfaction after the start of a meal. This effect is a result of peripheral and central GLP-1 receptor activation, and is afforded by the ability of GLP-1 RAs to cross the blood brain barrier (BBB) [130]. In one study, vagal deafferentation attenuated the anorectic effects of exendin-4 and liraglutide [130]. Another recent study showed that liraglutide can activate the GLP-1 receptor on appetite-regulating neurons in the arcuate nucleus (ARC), which may mediate its long-term weight loss effects[131]. Collectively, GLP-1 RAs show varied effects on weight loss, which may result from multiple factors. For example, unlike liraglutide, clinical studies with albiglutide resulted in little weight loss, potentially attributed to limited penetration of the large molecular weight of albumin fusion proteins into the central nervous system, the major site of action [132].

This work was supported by NIH HL134569 and Amneal Pharmaceuticals grant to A. Schwendeman. Emily E. Morin was supported by NIH T32 GM008353 and T32 HL125242.