3.1 Routes of drug administration and bioavailability

Drug administration routes play a crucial role in pharmacokinetics. From oral pills to intravenous injections, each method has unique pros and cons. Understanding these differences helps healthcare providers choose the best option for effective treatment.

Bioavailability is key in determining how much of a drug reaches its target. It varies widely between routes and affects dosing decisions. Factors like patient age, drug properties, and absorption rates all influence which route works best for a particular medication.

Drug Administration Routes: Advantages vs Disadvantages

Oral and Parenteral Routes

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• Oral administration provides convenience and cost-effectiveness but results in slower onset and susceptibility to first-pass metabolism
• Parenteral routes (intravenous, intramuscular, subcutaneous) offer rapid onset and high bioavailability but require invasive procedures and medical supervision
  • Intravenous administration bypasses absorption barriers, delivering drugs directly into the bloodstream
  • Intramuscular injections allow for depot formulations, providing sustained drug release (testosterone cypionate)
  • Subcutaneous injections suit drugs with small molecular weight and high potency (insulin)

Topical, Transdermal, and Inhalation Routes

• Topical and transdermal routes allow localized effects and controlled release but may have limited systemic absorption
  • Topical corticosteroids treat skin inflammation with minimal systemic side effects
  • Transdermal patches deliver drugs like nicotine or fentanyl over extended periods
• Inhalation provides rapid onset for respiratory conditions but requires special devices and patient education
  • Bronchodilators (albuterol) quickly relieve asthma symptoms
  • Inhaled corticosteroids (fluticasone) manage chronic obstructive pulmonary disease

Rectal Administration and Route Considerations

• Rectal administration bypasses first-pass metabolism but may be less socially acceptable and have variable absorption
  • Suppositories deliver drugs like diazepam for seizures when oral route unavailable
  • Rectal acetaminophen reduces fever in children who cannot swallow tablets
• Route selection impacts drug efficacy, patient compliance, and treatment outcomes
  • Intravenous antibiotics treat severe infections more rapidly than oral formulations
  • Long-acting injectable antipsychotics improve medication adherence in schizophrenia patients

Bioavailability: Importance in Drug Therapy

Bioavailability Concept and Measurement

• Bioavailability represents the fraction of administered drug dose reaching systemic circulation in its active form
• Expressed as a percentage, bioavailability determines effective drug dose
• Intravenous administration achieves 100% bioavailability, serving as reference for other routes
• Oral bioavailability often falls below 100% due to incomplete absorption and first-pass metabolism
  • Propranolol exhibits low oral bioavailability (~30%) due to extensive first-pass metabolism
  • Levothyroxine demonstrates high oral bioavailability (~80%) with minimal first-pass effect

Impact on Therapeutic Efficacy and Safety

• Bioavailability affects drug's therapeutic efficacy, onset of action, and potential toxicity
• Understanding bioavailability guides dose adjustments when switching between formulations or routes
  • Oral morphine requires higher doses than intravenous morphine due to lower bioavailability
  • Tacrolimus dosing differs between oral and intravenous formulations based on bioavailability differences
• Bioavailability variations influence drug plasma concentrations and therapeutic outcomes
  • Generic drugs must demonstrate bioequivalence to brand-name counterparts for FDA approval
  • Food interactions may alter drug bioavailability (grapefruit juice increases bioavailability of certain statins)

Factors Influencing Drug Administration Route

Patient-Specific Considerations

• Age, physical condition, and ability to self-administer impact route selection
  • Elderly patients may struggle with inhaler devices, necessitating alternative routes
  • Unconscious patients require parenteral or rectal administration
• Patient compliance and preferences influence adherence to treatment regimens
  • Once-daily extended-release formulations improve compliance compared to multiple daily doses
  • Some patients prefer oral medications over injections, affecting long-term adherence

Drug Properties and Therapeutic Goals

• Chemical stability, lipophilicity, and molecular size determine suitable administration routes
  • Peptide drugs (insulin) degrade in the gastrointestinal tract, requiring parenteral administration
  • Lipophilic drugs cross the blood-brain barrier more easily, affecting central nervous system effects
• Onset of action, duration of effect, and targeted site guide route selection
  • Sublingual nitroglycerin provides rapid relief for angina pectoris
  • Depot injections of antipsychotics maintain therapeutic levels for weeks or months

Pharmacokinetic and Practical Considerations

• Absorption rate, bioavailability, and elimination half-life influence route choice
  • Drugs with short half-lives may require extended-release formulations or frequent dosing
  • High first-pass metabolism may necessitate non-oral routes (transdermal fentanyl)
• Cost, availability of formulations, and administration frequency affect treatment decisions
  • Generic oral medications often cost less than branded injectable alternatives
  • Limited availability of certain formulations may restrict route options in some regions

Drug Absorption and Bioavailability: Relationship

Absorption Process and Bioavailability Impact

• Drug absorption moves drugs from administration site into bloodstream, directly influencing bioavailability
• Extent of absorption affects bioavailability, particularly for non-intravenous routes
  • Incomplete intestinal absorption reduces oral bioavailability of some antibiotics (tetracyclines)
  • Enhanced absorption through nasal mucosa increases bioavailability of intranasal vaccines
• Absorption rate and extent vary between administration routes, impacting bioavailability
  • Sublingual absorption bypasses first-pass metabolism, increasing bioavailability of nitroglycerin
  • Intramuscular injections provide slower absorption but higher bioavailability than subcutaneous route for some drugs

Factors Affecting Absorption and Bioavailability

• Drug solubility, membrane permeability, and blood flow to absorption site influence absorption
  • Lipophilic drugs (benzodiazepines) absorb more readily across cell membranes
  • Increased blood flow to gastrointestinal tract after meals can enhance absorption of some drugs
• Formulation factors modify absorption and consequently affect bioavailability
  • Micronization increases surface area, enhancing dissolution and absorption of poorly soluble drugs
  • Enteric coatings protect drugs from stomach acid, altering absorption site and bioavailability
• Physiological barriers impact drug absorption and resultant bioavailability
  • Gastric pH affects ionization and absorption of weak acids and bases
  • Intestinal efflux transporters (P-glycoprotein) reduce absorption of certain drugs