Pharmacology of Bronchodilators

1. Beta-2 (β2) Adrenergic Agonists
2. Anticholinergic Agents (Muscarinic Receptor Antagonists)
3. Methylxanthines (Theophylline)

1. Beta-2 Adrenergic Agonists

Chemistry

Classification

Mode of Action

• β2 receptor agonism activates adenylyl cyclase via Gs protein
• Increased intracellular cAMP
• cAMP activates protein kinase A (PKA)
• PKA phosphorylates myosin light chain kinase (MLCK) → reduces its activity → smooth muscle relaxation
• PKA also opens K+ channels (BKCa) → membrane hyperpolarization → relaxation
• Reduces mediator release from mast cells
• Increases mucociliary clearance

Side Effects

• Muscle tremor (β2 receptors in skeletal muscle)
• Tachycardia (β1 stimulation at high doses)
• Hypokalemia (K+ uptake into cells via Na+/K+-ATPase activation)
• Metabolic effects: hyperglycemia, increased free fatty acids
• Tolerance: receptor downregulation with prolonged use

Key Clinical Points

• Inhaled SABAs (e.g., salbutamol) = first-line reliever in asthma
• LABAs should never be used as monotherapy in asthma (risk of fatal exacerbations); always combined with inhaled corticosteroids (ICS)
• LABAs + ICS combination is the standard step-up therapy in asthma

2. Anticholinergic Agents (Muscarinic Receptor Antagonists)

Mechanism

• Block muscarinic (M3) receptors on airway smooth muscle
• Inhibit vagally mediated bronchoconstriction
• Reduce mucus secretion (via M1/M3 on submucosal glands)
• Less potent bronchodilators than β2 agonists in asthma, but equally or more effective in COPD (where cholinergic tone is the dominant reversible component)

Classification

Side Effects

• Dry mouth (most common)
• Urinary retention (caution in BPH)
• Constipation
• Glaucoma (if inhaled drug contacts eyes - use mouthpiece correctly)
• Minimal systemic anticholinergic effects due to poor absorption

3. Methylxanthines (Theophylline/Aminophylline)

Mechanism (Multiple)

1. PDE (phosphodiesterase) inhibition - particularly PDE3 and PDE4 → increased cAMP and cGMP → smooth muscle relaxation
2. Adenosine receptor antagonism (A1 and A2 receptors) - contributes to bronchodilation and anti-inflammatory effects
3. Histone deacetylase (HDAC) activation at low doses - anti-inflammatory effects, restores corticosteroid sensitivity

Pharmacokinetics

• Oral bioavailability ~100% (slow-release preparations preferred)
• Metabolized by CYP1A2 (primarily) and CYP3A4
• Narrow therapeutic index: target plasma concentration 5-15 mg/L (10-20 mg/L previously used but toxic)
• Half-life varies widely (3-13 h) depending on age, smoking status, disease, and drug interactions

Factors Affecting Clearance

Side Effects (dose-related, >15 mg/L)

• Nausea, vomiting, headache (PDE4 inhibition) - most common
• Abdominal discomfort, restlessness
• Increased acid secretion, diuresis (adenosine A1 antagonism)
• Cardiac arrhythmias (PDE3 inhibition + A1 antagonism) at high concentrations
• Seizures at very high concentrations (central A1 antagonism)
• Behavioral disturbances in children

Clinical Use

• Acute asthma: IV aminophylline is less effective than nebulized β2 agonists; reserved for refractory cases
• COPD: Third-line bronchodilator, added to anticholinergics + β2 agonists in severe disease
• Low-dose theophylline (5-10 mg/L) as add-on to ICS in mild persistent asthma (less expensive than LABAs)

Novel/Emerging Bronchodilators

• Bitter taste receptor (TAS2R) agonists - activate TAS2R on airway smooth muscle → bronchodilation independent of cAMP
• Rho-kinase inhibitors - target Rho-kinase pathway downstream of Ca2+ sensitization
• E-prostanoid (EP2/EP4) receptor agonists - prostaglandin-based bronchodilation
• These are still investigational

Comparison Summary

• Goodman & Gilman's Pharmacological Basis of Therapeutics, 16th ed. - Chapter 44 (Pulmonary Pharmacology), pp. 898-904
• Katzung's Basic and Clinical Pharmacology, 16th ed. - Chapter on Bronchodilators

Conservation of Medicinal Plants

Why It Matters

Major Threats to Medicinal Plants

Conservation Strategies

1. In-Situ Conservation

• Protected areas: National parks, wildlife sanctuaries, biosphere reserves, forest reserves
• Community-managed reserves: Local and indigenous communities manage and patrol areas - this is often the most effective approach because it aligns economic incentives with conservation
• Sacred groves: Traditional practice in India, Africa, and Southeast Asia where forest patches are protected by cultural/religious taboos
• Agroforestry: Integrating medicinal plants into farm systems to reduce pressure on wild populations while maintaining ecological relationships
• Key Biodiversity Areas (KBAs): IUCN-designated sites identified as irreplaceable for biodiversity, prioritized for protection

2. Ex-Situ Conservation

3. Sustainable Harvesting Practices

• Harvest only a defined fraction of the population (typically <30% of above-ground biomass)
• Rotational harvesting - allowing recovery periods
• Prefer leaves and flowers over roots and bark (less destructive)
• Avoid harvesting during flowering/fruiting (reproductive seasons)
• Use ethical wildcrafting guidelines (e.g., United Plant Savers protocols)
• Promote cultivation over wild collection wherever possible (domestication programs)

4. Cultivation and Domestication

• Organic farming and permaculture of high-demand medicinal species
• Reduces wild-harvesting pressure
• Allows standardization of phytochemical content (important for pharmaceutical use)
• Agroforestry models integrate medicinal crops with shade trees
• Key crops under cultivation programs: Withania somnifera, Tinospora cordifolia, Ocimum sanctum, Aloe vera, Glycyrrhiza glabra

5. Ethnobotanical Documentation

• Systematic recording of traditional plant knowledge from indigenous healers and communities
• Creates herbal monographs and databases (e.g., WHO Monographs on Selected Medicinal Plants)
• Protects against biopiracy (unauthorized commercial use of traditional knowledge)
• Tools: participatory rural appraisal, field ethnobotanical surveys, digital databases (MPNS - Medicinal Plant Names Service, IUCN)

6. Policy and Legal Frameworks

7. Community-Based Conservation and Economic Incentives

• Medicinal plants have high economic value - this can be harnessed for conservation
• Secondary forests that provide income to local communities (from medicinal plant harvesting) act simultaneously as carbon sinks
• Payments for Ecosystem Services (PES), eco-certification schemes, and fair-trade herbal products incentivize protection
• The 2024 study on indigenous medicinal plants (Mbelebele et al., Sustainability) showed that conservation programs for indigenous medicinal plants significantly improved economic well-being of smallholder farmers in South Africa

8. Climate-Adaptive Strategies (Emerging Focus)

• Map shifts in species distribution under climate scenarios
• Prioritize assisted migration/translocation of range-restricted species
• Integrate climate models into protected area planning
• Promote climate-resilient cultivation systems (agroforestry, permaculture)
• Recognize that cultivation alone cannot replace conservation if overexploitation and habitat threats are not addressed simultaneously

Key Endangered Medicinal Plants (Global Examples)

Summary: Integrated Conservation Framework

         ASSESS                    PLAN                    ACT
   (Red List, surveys,    (KBAs, strategies,        (Protected areas,
   ethnobotany, climate   policy, ABS laws,         cultivation, seed
   modeling)              community programs)        banks, reintroduction)

Pharmacology of Antithyroid Drugs

1. Thioamides (Thioureylenes) - PTU and Methimazole
2. Iodine and Iodides (including radioactive iodine)
3. Ionic Inhibitors
4. Adjunct drugs (beta-blockers, lithium)

Background: Thyroid Hormone Biosynthesis (Target Steps)

1. Iodide uptake into thyroid follicle cells via sodium/iodide symporter (NIS)
2. Transport across the apical membrane via pendrin
3. Oxidation of I⁻ to I₂ by thyroid peroxidase (TPO)
4. Organification: iodination of tyrosine residues on thyroglobulin → MIT, DIT
5. Coupling: MIT + DIT → T3; DIT + DIT → T4 (also catalyzed by TPO)
6. Hydrolysis of thyroglobulin → release of T3 and T4
7. Peripheral conversion of T4 → T3 by 5'-deiodinase

1. Thioamides (Thioureylenes)

Mechanism of Action

• Inhibit thyroid peroxidase (TPO) - block oxidation of iodide and the organification of iodine (incorporation into tyrosine residues of thyroglobulin)
• Inhibit coupling of iodotyrosyl residues (MIT + DIT → T3/T4)
• PTU additionally inhibits peripheral conversion of T4 → T3 by inhibiting type 1 deiodinase (5'-deiodinase) - methimazole does NOT have this effect

This extra action of PTU makes it the preferred drug in thyroid storm and in the first trimester of pregnancy (where rapid control is needed and teratogenicity of MMI - aplasia cutis - is a concern).

Pharmacokinetics Comparison

Clinical Uses

• Graves' disease (first-line medical therapy, especially in younger patients)
• Preparation for thyroidectomy (render patient euthyroid pre-op)
• Thyroid storm - PTU preferred (blocks peripheral T4→T3 conversion)
• First trimester of pregnancy - PTU preferred (MMI associated with aplasia cutis, choanal/esophageal atresia)
• Second/third trimester - switch to MMI (PTU has rare severe hepatotoxicity risk)
• Control of hyperthyroidism before radioiodine therapy

Adverse Effects

Agranulocytosis is the most feared adverse effect. Patients must be instructed to stop the drug and seek immediate medical attention if they develop fever, sore throat, or mouth ulcers. Routine monitoring of CBC is not reliably predictive as it occurs suddenly.

2. Iodine and Iodides (Stable Iodine)

Mechanism

• Wolff-Chaikoff effect: Excess iodine acutely inhibits its own organification, transiently reducing T3/T4 synthesis
• Inhibits thyroid hormone release from thyroglobulin (main therapeutic effect)
• Reduces vascularity of the thyroid gland (important pre-operatively)
• Reduces gland size and firmness

Preparations

• Lugol's iodine (5% iodine + 10% KI solution)
• Saturated solution of potassium iodide (SSKI)

Clinical Uses

• Pre-operative preparation for thyroidectomy (given 10-14 days before surgery to reduce vascularity and bleeding risk) - used in combination with thioamide
• Thyroid storm (rapid reduction of hormone release)
• After radioiodine therapy (temporary)

Limitations

• Escape phenomenon: Thyroid "escapes" the Wolff-Chaikoff effect after 1-2 weeks - cannot be used as long-term monotherapy
• Iodine-Basedow effect: In iodine-deficient patients with nodular goiter, iodine may paradoxically worsen hyperthyroidism
• Contraindicated if radioiodine therapy is planned soon (competes with ¹³¹I uptake)

3. Radioactive Iodine (¹³¹I)

Mechanism

• ¹³¹I has a t½ of 8 days and emits both β particles (therapeutic - tissue destruction) and γ rays (diagnostic imaging)
• Taken up by thyroid follicle cells via NIS (same mechanism as stable iodine)
• β particles destroy thyroid parenchymal cells with minimal damage to surrounding tissue

• 99% of radiation expended within 56 days

Isotopes

Indications

• Hyperthyroidism in older patients and those with heart disease (clearest indication)
• Graves' disease that has recurred after thyroidectomy or failed antithyroid drugs
• Toxic nodular goiter / toxic adenoma
• Thyroid cancer - ablation of residual tissue post-thyroidectomy; treatment of metastases

Dosing

• Usual total dose: 4-15 mCi (target ~8 mCi delivered to thyroid)
• Based on 24-h radioiodine uptake and estimated gland size
• Symptoms improve over 2-3 months

Contraindications

• Pregnancy (absolute - causes fetal hypothyroidism)
• Breastfeeding (absolute)
• Planned pregnancy within 6 months
• Active thyroid eye disease (can worsen orbitopathy in Graves')

Complications

• Hypothyroidism - most common long-term outcome (nearly universal over years); requires lifelong levothyroxine
• Radiation thyroiditis - transient worsening of hyperthyroidism post-treatment
• Neck pain/tenderness
• Rare: Thyroid storm (if pre-treatment thioamide not given)

4. Ionic Inhibitors (Anion Inhibitors)

• Perchlorate (ClO₄⁻) - most potent; rarely used now due to aplastic anemia risk
• Thiocyanate (SCN⁻) - weaker; found naturally in cruciferous vegetables
• Pertechnetate (TcO₄⁻) - used diagnostically (⁹⁹ᵐTc scanning); also blocks NIS

5. Adjunct Drugs

Beta-Blockers (Propranolol)

• Mechanism: Block β-adrenergic effects of excess thyroid hormone (tachycardia, tremor, anxiety, heat intolerance); propranolol also partially inhibits peripheral T4→T3 conversion at high doses
• Use: Rapid symptomatic relief while waiting for thioamides to take effect; thyroid storm; pre-operatively
• Do NOT reduce thyroid hormone synthesis/secretion - purely symptomatic

Lithium

• Inhibits thyroid hormone release from thyroglobulin (similar mechanism to iodide)
• Rarely used; mainly when patient cannot take iodides
• Toxic and narrow therapeutic index - limits routine use

Glucocorticoids (high dose)

• Used in thyroid storm - inhibit peripheral T4→T3 conversion and reduce hormone release
• Dexamethasone 2 mg q6h or hydrocortisone IV is standard in thyroid storm protocol

Comparison: Choice of Antithyroid Drug

Thyroid Storm Management (Summary)

1. PTU 500-1000 mg loading dose, then 250 mg q4h (blocks synthesis + conversion)
2. Lugol's iodine / SSKI given 1 hour after PTU (blocks release; must give PTU first to prevent new hormone synthesis from the iodine load)
3. Propranolol IV/oral (blocks sympathetic hyperactivity)
4. Dexamethasone 2 mg IV q6h (blocks T4→T3 conversion, suppresses adrenal stress response)
5. Supportive care: cooling, IV fluids, treat precipitating cause

• Goodman & Gilman's The Pharmacological Basis of Therapeutics, 16th ed. - Chapter 47: Thyroid and Antithyroid Drugs, pp. 965-973
• Katzung's Basic and Clinical Pharmacology, 16th ed. - Chapter 38: Thyroid & Antithyroid Drugs, pp. 1079-1090