Comprehensive Study Notes on Anti-Diabetic Drugs and Insulin

Reference Material and Sources

  • Primary Academic Sources:
    • Bertram G. Katzung Basic & Clinical Pharmacology, 15TH15^{TH} Edition.
    • Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13TH13^{TH} Edition.

Learning Objectives and Focus Areas

  • Normal Physiology: Identification of healthy endocrine function.
  • Pathophysiology: Targeted understanding of the disease states and associated morbidity of Diabetes Mellitus.
  • Drug Targets: Identification of potential biochemical and physiological targets for pharmacological intervention.
  • Pharmacokinetics (PK) and Pharmacodynamics (PD): Detailed study of how the body processes anti-diabetic drugs and how the drugs effect the body.
  • Clinical Pharmacology: Application of these agents in the management of diabetes complications and routine care.

The Endocrine Pancreas and Hormonal Regulation

  • Hormonal Components of the Endocrine Pancreas:
    • Insulin: The primary anabolic hormone regulating glucose levels.
    • Islet Amyloid PolyPeptide (IAPP): Co-secreted with insulin; also known as amylin.
    • Glucagon: A catabolic hormone that opposes insulin by raising blood glucose.
    • Somatostatin: A regulatory hormone that inhibits the release of both insulin and glucagon.
    • Pancreatic Peptide: Involved in the regulation of exocrine and endocrine pancreatic secretion.
    • Ghrelin: Primarily produced in the stomach but also present in the pancreas; involved in growth hormone secretion and appetite regulation.

Molecular Structure of Insulin

  • Chain Composition:
    • A Chain: Consists of 2121 amino acids.
    • B Chain: Consists of 3030 amino acids.
    • Connecting Peptide (C-peptide): A fragment cleaved from proinsulin to form active insulin. It consists of a sequence including Gly, Ile, Val, Glu, Gln, Cys, Cys, Thr, Ser, Ile, Cys, Ser, Leu, Tyr, Gln, Leu, Glu, Asn, Tyr, and Cys.
  • Linkages: The A and B chains are joined by disulfide (S−SS-S) bridges. Specifically, there are disulfide bonds at positions A7−B7A7-B7 and A20−B19A20-B19, along with an intrachain disulfide bond on the A chain at A6−A11A6-A11.
  • Processing: Proinsulin is converted to insulin and C-peptide through proteolysis by prohormone convertases. In the diagram, residues like $Arg(31)$, $Arg(32)$, $Lys(64)$, and $Arg(65)$ mark cleavage points.

Regulation of Insulin Secretion

  • Stimulatory Factors:
    • Monosaccharides: Glucose and mannose.
    • Amino Acids: Specifically leucine, arginine, and other gluconeogenic amino acids.
    • Incretin Hormones: Glucagon-like Polypeptide 1 (GLP-1) and Glucose-dependent insulinotropic polypeptide (GIP).
    • Pancreatic/Gastrointestinal Hormones: Glucagon and Cholecystokinin.
    • Lipids: High concentrations of fatty acids.
    • Neuronal Activity: β\beta-adrenergic sympathetic activity.
  • Inhibitory Factors:
    • Feedback/Paracrine Inhibition: Insulin, Islet amyloid polypeptide (IAPP), and Somatostatin.
    • Hormones: Leptin.
    • Neuronal Activity: α\alpha-adrenergic sympathetic activity.
    • Metabolic States: Chronically elevated glucose (leading to desensitization) and low concentrations of fatty acids.

Cellular Mechanism of Insulin Release

  1. Glucose Entry: Glucose enters the pancreatic β\beta-cell via the GLUT2 glucose transporter.
  2. Metabolism: Glucose is metabolized, increasing the intra-cellular ratio of ATP to ADP.
  3. Channel Closure: Elevated ATP levels close the ATP-sensitive K+K^+ channels.
  4. Depolarization: The closure of K+K^+ channels leads to cell depolarization.
  5. Calcium Influx: Depolarization opens voltage-gated Ca2+Ca^{2+} channels, leading to an influx of Ca2+Ca^{2+}.
  6. Exocytosis: The increase in intracellular Ca2+Ca^{2+} triggers the exocytosis of insulin-containing storage granules.
  7. Pharmacological Modulation: Sulfonylurea drugs act by binding to the sulfonylurea receptor component of the K+K^+ channel, effectively blocking the channel to induce depolarization and insulin release independent of glucose levels.

Insulin Mechanism of Action: The Receptor

  • Receptor Structure: A heterotetramer consisting of two α\alpha subunits (extracellular, binding site) and two β\beta subunits (spanning the membrane and containing tyrosine kinase domains).
  • Signaling Pathways:
    • Phosphatidylinositol-3 Kinase (PI-3K) Pathway: Activates IRS (Insulin Receptor Substrates) which leads to the synthesis of lipids, proteins, and glycogen. It also promotes cell survival and triggers the translocation of GLUT4 vesicles to the cell membrane for glucose uptake.
    • MAP Kinase Pathway: Leads to cell growth, proliferation, and gene expression.

Physiological Effects of Insulin by Tissue Type

Liver Cells
  • Carbohydrate Metabolism: Decreased gluconeogenesis and glycogenolysis; increased glycolysis and glycogenesis.
  • Lipid Metabolism: Increased lipogenesis; decreased lipolysis.
  • Protein Metabolism: Decreased protein breakdown.
Muscle Tissue
  • Carbohydrate Metabolism: Increased glucose uptake (via GLUT4), increased glycolysis, and increased glycogenesis.
  • Protein Metabolism: Increased amino acid uptake and increased protein synthesis.
Fat Cells (Adipocytes)
  • Carbohydrate Metabolism: Increased glucose uptake and glycerol synthesis.
  • Lipid Metabolism: Increased synthesis of triglycerides and fatty acids; decreased lipolysis.

Diabetes Mellitus: Definition and Pathophysiology

  • General Definition: A condition characterized by elevated blood glucose (hyperglycemia) associated with absent or inadequate pancreatic insulin secretion, with or without concurrent impairment of insulin action.
  • Classification:
    • Type 1 DM: Absolute insulin deficiency due to β\beta-cell destruction.
    • Type 2 DM: Progressive insulin secretory defect on the background of insulin resistance.
    • Gestational DM: Diabetes diagnosed during pregnancy.

Pharmacological Strategies for Lowering Blood Glucose

  1. Force Glucose into Cells: Increasing peripheral uptake.
  2. Increase Insulin Content: Through exogenous insulin or insulin secretagogues.
  3. Increase Insulin Sensitivity: Reducing resistance at the receptor or post-receptor level.
  4. Decrease Glucose Absorption: Modifying the rate of carbohydrate breakdown in the gut or reabsorption in the kidneys.

Parenteral Anti-Diabetic Drugs: Insulin

Classification by Action Duration
  • Rapidly Acting:
    • Agents: Lispro, Aspart, Glulisine.
    • Onset: 5−155-15 minutes.
    • Peak: 1−1.51-1.5 hours.
    • Duration: 3−43-4 hours.
  • Short Acting:
    • Agents: Regular Human Insulin.
    • Onset: 30−6030-60 minutes.
    • Peak: 22 hours.
    • Duration: 6−86-8 hours.
  • Intermediate Acting:
    • Agents: Human NPH (Neutral Protamine Hagedorn).
    • Onset: 2−42-4 hours.
    • Peak: 6−76-7 hours.
    • Duration: 10−2010-20 hours.
  • Long Acting:
    • Insulin Glargine: Soluble, peakless (flat profile). Duration: ∼24\sim 24 hours.
    • Insulin Detemir: Flat profile. Duration: 1717 hours.
    • Insulin Degludec: Contains Zinc and phenol. Duration: >42>42 hours.
Inhaled Insulin
  • Technosphere inhaled insulin: Onset 5−155-15 minutes, Peak 11 hour, Duration 33 hours.
Premixed Insulin Formulations
  • 7070 NPH / 3030 regular (e.g., Novolin, Humulin).
  • 75/2575/25 NPL / Lispro (Humalog mix).
  • 50/5050/50 NPL / Lispro.
  • 70/3070/30 NPA / Aspart (Novolog mix).
  • 70/3070/30 Degludec / Aspart (Ryzodeg).

Clinical Use and Complications of Insulin

  • Indications: Type 1 DM, Type 2 DM (routine or emergency), Gestational DM, Diabetic Ketoacidosis (DKA), Hyperosmolar Hyperglycemic Syndrome (HHS).
  • Methods of Delivery:
    • Insulin syringes and needles.
    • Insulin Pens.
    • Continuous Subcutaneous Insulin Infusion (CSII) / Insulin Pumps.
    • Inhaled Insulin.
  • Adverse Effects:
    • Hypoglycemia: The most common and serious complication.
    • Immunogenicity: Development of insulin antibodies.
    • Lipodystrophy: Localized fat atrophy or hypertrophy at injection sites.
    • Weight Gain.

Non-Insulin Parenteral Agents

Incretin Mimetics / GLP-1 Receptor Agonists
  • Mechanism: Mimic the GLP-1 hormone to increase insulin release and decrease glucagon release.
  • Agents: Exenatide, Exenatide LAR, Liraglutide (3 mg3\,mg dose specifically for weight loss), Semaglutide, Albiglutide, Dulaglutide.
  • Adverse Effects: Nausea, Pancreatitis, link to Thyroid C-cell tumors.
Amylin Analog (Pramlintide)
  • Mechanism: Synthetically modified IAPP (proline substitutions at positions 25,28, and 2925, 28, \text{ and } 29). It decreases glucagon release, slows gastric emptying, increases satiety, and decreases food intake.

Oral Anti-Diabetic Agents: Insulin Secretagogues

Sulfonylureas
  • First Generation: Tolbutamide, Chlorpropamide, Tolazamide, Acetohexamide.
  • Second Generation: Glyburide (Glibenclamide), Glipizide, Glimepiride, Gliclazide.
  • Considerations: Metabolism and drug interactions (e.g., aspirin potentiating effects).
Meglitinide Analogs and D-Phenylalanine Derivatives
  • Agents: Repaglinide, Mitiglinide (Meglitinides); Nateglinide (D-phenylalanine derivative).
  • Mechanism: Similar to sulfonylureas but with faster onset and shorter duration, targeting postprandial glucose.

Oral Anti-Diabetic Agents: Biguanides (Metformin)

  • Trade Name: Glucophage.
  • Mechanism: Activates AMPK (AMP-activated protein kinase) in the liver to inhibit gluconeogenesis (specifically blocking pyruvate carboxylase) and improve glucose utilization in peripheral tissues.
  • Adverse Clinical Link: Lactic Acidosis. Risk increases with dehydration or acute kidney injury. Pathophysiology involves the accumulation of lactic acid because the first step of gluconeogenesis (pyruvate to oxaloacetate) is blocked.

Oral Anti-Diabetic Agents: Thiazolidinediones (TZDs)

  • Agents: Pioglitazone, Rosiglitazone.
  • Mechanism: Ligands for Peroxisome Proliferator-activated receptor gamma (PPAR−γPPAR-\gamma), a nuclear receptor.
  • Effects:
    • Increased expression of GLUT1 and GLUT4.
    • Decreased free fatty acid levels and hepatic glucose output.
    • Increased adiponectin; decreased resistin release from adipocytes.
    • Increased differentiation of preadipocytes to adipocytes.

Other Oral Classes

  • DPP-4 Inhibitors (Gliptins): Sitagliptin, Saxagliptin, Linagliptin, Vildagliptin, Alogliptin. They inhibit the Dipeptidyl Peptidase-4 enzyme, prolonging the half-life of native incretins (GLP-1 and GIP).
  • α\alpha-Glucosidase Inhibitors: Acarbose, Miglitol, Voglibose. These act in the small intestine brush border to slow the breakdown of oligosaccharides into monosaccharides, reducing postprandial hyperglycemia.
  • SGLT-2 Inhibitors: Canagliflozin, Dapagliflozin, Empagliflozin. They block the Sodium-glucose co-transporter 2 in the kidney, preventing the reabsorption of glucose into the bloodstream.
  • Miscellaneous: Colesevelam hydrochloride, Bromocriptine.

Clinical Management Algorithm for Type 2 DM

  1. Initial Phase: Assessment of HbA1c, Diabetes Education, Medical Nutrition Therapy, Physical Activity.
  2. First Line: Metformin.
  3. Step-up Therapy: If A1c goals are not met, add a second agent (Sulfonylurea, TZD, DPP-4 inhibitor, GLP-1 RA, etc.).
  4. Triple Therapy: Metformin + two other agents.
  5. Advanced Therapy: Metformin + Insulin.
  6. Ongoing Care: Screening for complications (retinal exam, microalbuminuria, neuropathy, vascular evaluation) and treating comorbidities (dyslipidemia, hypertension, obesity, CV disease).

Questions & Discussion

  • Q1: Different types of Insulin.
    • Task: Enumerate types with examples and draw action graphs.
    • Examples: Rapid (Aspart), Short (Regular), Long (Glargine).
  • Q2: Complications of Insulin Therapy.
    • Response: Hypoglycemia (managed with glucose/glucagon), Lipodystrophy (managed by rotating injection sites), and Immunogenicity.
  • Case 47: 58-year-old man, Type 1 DM, started on propranolol, severe hypoglycemia without warning signs like tremors.
    • Correct Mechanism: C. Masking of adrenergic symptoms of hypoglycemia. β\beta-blockers mask the sympathetic response to low blood sugar.
  • Case 48: 22-year-old woman with DKA. Most appropriate insulin for IV infusion?
    • Correct choice: D. Regular insulin. It is the standard for intravenous use in acute settings.
  • Case 49: 67-year-old on metformin with dehydration and AKI. Most concerning adverse effect?
    • Correct choice: C. Lactic acidosis.
  • Case 50: Elderly patient on glibenclamide develops sweating/confusion after high-dose aspirin.
    • Explanation: C. Aspirin potentiates sulfonylurea-induced hypoglycemia.
  • Case 51: Diabetic on metformin scheduled for contrast-enhanced CT.
    • Management: C. Temporarily withhold metformin. This prevents lactic acidosis risk if the contrast media induces kidney dysfunction.