Acarbose

Table 1. Basic Profile of acarbose

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Drug Information

Table 2. Chemical and Physical Properties (DrugBank)

Drug Target

Acarbose is an orally active antidiabetic drug that acts by inhibiting carbohydrate digestive enzymes, (alpha glucosidases and alpha amylases). Through competitive and reversible inhibition of glucosidases, acarbose delays gastrointestinal digestion of carbohydrates, which in turn mitigates postprandial hyperglycemia (increase in blood glucose levels following a meal). Because its mechanism of action does not enhance insulin secretion, acarbose may be used in combination with other hyperglycemic agents, e.g. sulfonylureas, insulin or metformin, in order to enhance glycemic control (Drugs.com).

Acarbose inhibits human pancreatic alpha amylase (HPA) and membrane-bound intestinal alpha glucosidases, such as maltase glucoamylase (MGAM) and sucrase isomaltase (SI). Here we describe the complex of MGAM and acarbose.

Learn more about the targets of acarbose here.

Drug-Target Complex

Each molecule of the maltase glucoamylase (MGAM) enzyme is made up of 1867 residues and contains two homologous catalytic subunits. The N-terminal domain (residues 1-868) is called NtMGAM and C-terminal domain (residues 955-1867) is called CtMGAM. Both subunits carry out the same catalytic reaction - hydrolysis of α-(1,4) linked glucose units in linear oligosaccharide substrates yeilding glucose monomers. However, they differ in their distinct specificity for varying lengths of malto-oligosaccharides (Sim et al., 2008).

Both the NtMGAM and CtMGAM domains are comprised of five major sub-domains:

• a trefoil Type-P domain
• an N-terminal β-sandwich domain
• a catalytic (β/α)8 barrel domain with two inserted loops
• a proximal C-terminal domain
• a distal C-terminal domain

The tertiary structures of NtMGAM and CtMGAM reveal a similar fold (Figure 2), however the latter has a 21-residue insert (shown in cyan, Figures 2b and 3b), and gives it a larger binding pocket to bind longer substrates than NtMGAM.

Figure 2. Ribbon diagram representation of MGAM in complex with acarbose, with individual structural subdomains highlighted in different colors (a) NtMGAM (PDB ID 2qmj; Sim et al., 2008). (b) CtMGAM (PDB ID 3top; Ren et al., 2011). CtMGAM has 21 additional residues in the catalytic domain, highlighted by the cyan ribbon. Acarbose is shown in ball and stick representation, color-coded by atom type (C: green; N: blue; O: red).

Acarbose is a substrate-like inhibitor of HPA, MGAM and SI. It has a tetrasaccharide-like structure with an acarviosine group α-linked to a maltose. The first two rings of the non-reducing end of acarbose (also known as acarvosine) occupy the −1 and +1 sugar subsites of the active site, with the N-linked bond occupying the catalytic center. Substrate cleavage occurs between the -1 and +1 subsites. A close up view of the active site shows acarbose as a stick figure (Figure 3).

Figure 3. Close up view of the active site of (a) NtMGAM (PDB ID 2qmj; Sim et al., 2008). (b) CtMGAM (PDB ID 3top; Ren et al., 2011). Acarbose is shown as a stick figure, color-coded by atom type (C: green; N: blue; O: red). The sugar subsites -1, +1 are shown in pink and blue arcs respectively.

Close examination of the structure reveals that acarbose binds to NtMGAM via extensive hydrogen bonding interactions, represented as blue lines in Figure 4a with active site residues Asp203, Thr205, Asp327, Arg526, Asp542, and His600 and surrounding water molecules. The two rings from the reducing end of acarbose make minimal interactions with active site residues. A similar hydrogen bonding network is observed (Figure 4b) between acarbose and active site residues of CtMGAM (Asp1157, Asp1279, Arg1510, Asp1526, and His1584). The difference between acarbose and a α-(1,4) linked tetrasaccharide substrate is a nitrogen atom in place of an oxygen atom. The catalytic nucleophile Asp (Asp443 in NtMGAM/Asp1420 in CtMGAM) does not interact with acarbose, whereas the same residue forms a covalent intermediate with oligosaccharide substrates and plays a major role in hydrolyzing linear oligosaccharides to monomers. The acid/base residue Asp542 (in NtMGAM) and Asp1526 (in CtMGAM) makes several hydrogen bonds with the drug molecule. These structural studies have revealed that acarbose acts by occluding the active sites of HPA, MGAM, and SI, preventing binding of oligosaccharide substrates, and delaying carbohydrate digestion.

Figure 4. (a) Hydrogen bonding interactions (blue lines) between acarbose (stick figure) and enzyme residues in (a) NtMGAM (PDB ID 2qmj; Sim et al., 2008); (b) CtMGAM (PDB ID 3top; Ren et al., 2011). The catalytic nucleophile Asp is labeled in red and the acid/base residue Asp in blue and shown in ball and stick representation, while other enzyme residues are shown as stick figures. The acarvosine ring occupies the subsites of the enzyme where the −1 and +1 sugars of the substrate bind.

Pharmacologic Properties and Safety

Table 3. Pharmacokinetics: ADMET of acarbose

Low oral bioavailability of the parent drug accounts for the minimal systemic effects of the drug. Acarbose is broken down into glucose, maltose, and acarvosine in the intestine. Roughly 34% of these metabolites are absorbed and removed from the blood via renal elimination. Acarbose molecules that are not metabolized simply pass through the gastrointestinal tract and are excreted.

Drug Interactions and Side Effects

Acarbose does not display harmful effects on cardiac health, nor is it carcinogenic (DrugBank). According to the National Institutes of Health, 2-5% of patients in a case study displayed elevated serum liver enzyme levels. The levels were three times the upper limit in these patients. However, serum enzyme levels returned to normal when acarbose therapy was discontinued and no damaging effects on the liver were observed. Following approval of acarbose, there were at least a dozen cases of clinically apparent liver injury linked to acarbose use. Adverse effects on hepatic health from administration of acarbose were unexpected and remain unexplained, since acarbose is minimally absorbed due to its oligosaccharide chemical structure (NIH).

Table 4. Drug Interactions and Side Effects of acarbose

Because acarbose is not extensively metabolized in the liver, significant drug-drug interactions are rare. However, there have been case reports documenting reduced absorption of digoxin, an antiarrhythmic and blood pressure reducer, and an increase in absorption of warfarin, a blood thinner. In combination therapy with other hypoglycemic agents, acarbose tablets did not interfere with the absorption or disposition of glyburide (sulfonylureas drug class) in diabetic patients (Drugs.com)

Regulatory Approvals/Commercial

Developed by Bayer Health Care Pharmaceuticals Inc. and approved by the FDA on September 6, 1995, Precose (acarbose) can be used as a monotherapy or as an adjunct to other antidiabetic medications, including sulfonylureas, insulin, and metformin (Drugs.com). It has been reported that acarbose mitigates the weight-increasing effect of sulfonylureas. Precose is available in 25, 50 and 100 mg tablets.

Links

Table 5. Links to Relevant Resources

References

Acarbose, National Institutes of Health. https://livertox.nlm.nih.gov/Acarbose.htm

Acarbose, Drugs.com. http://www.drugs.com/mtm/acarbose.html.

Acarbose, DrugBank. https://www.drugbank.ca/drugs/DB00284

Ren, L., Qin X., Cao, X., Wang, L., Bai, F., Bai, G., and Shen, Y. (2011) Structural insight into substrate specificity of human intestinal maltase-glucoamylase. Protein Cell 2, 827-36. http://10.0.3.239/s13238-011-1105-3

Sim, L., Quezada-Calvillo, R., Sterchi, E.E., Nichols, B.L., and Rose, D.R. (2008). Human Intestinal Maltase–Glucoamylase: Crystal Structure of the N-Terminal Catalytic Subunit and Basis of Inhibition and Substrate Specificity. Journal of Molecular Biology 375, 782-92. http://www.sciencedirect.com/science/article/pii/S0022283607014271

Triplitt, C. (2006) Drug Interactions of Medications Commonly Used in Diabetes. Diabetes Spectrum 19, 202-211. http://spectrum.diabetesjournals.org/content/diaspect/19/4/202.full.pdf

August 2019, Christopher Markosian, Jennifer Jiang, and Dr. Sutapa Ghosh ; Reviewed by Dr. Stephen K. Burley
http://dx.doi.org/10.2210/rcsb_pdb/GH/DM/drugs/gi/acarbose