Section 4.10: Clinical Pharmacology: Infectious Diseases & Antimicrobial Stewardship

Clinical Pharmacology: Infectious Diseases & Antimicrobial Stewardship

Infectious disease pharmacotherapy requires understanding target pathogens, drug penetration, resistance patterns, and regulatory constraints. In Saudi Arabia, the Saudi Food and Drug Authority (SFDA) enforces strict regulations on antibiotic dispensing to support global antimicrobial stewardship.

---

1. Pneumonia Management

Community-Acquired Pneumonia (CAP)

CAP is diagnosed in patients who have not been hospitalized or in a long-term care facility within 14 days of symptom onset. The primary pathogens are Streptococcus pneumoniae, Haemophilus influenzae, and atypical pathogens (Mycoplasma pneumoniae, Chlamydophila pneumoniae).

• Outpatient Therapy:
  • Healthy patients (no comorbidities): Amoxicillin (1g TID) OR Doxycycline OR Macrolide (Clarithromycin or Azithromycin, if local pneumococcal resistance is < 25%).
  • Comorbidities (heart, lung, liver, or renal disease, diabetes, alcoholism, malignancy, asplenia): Monotherapy with a respiratory fluoroquinolone (Levofloxacin 750mg daily, Moxifloxacin 400mg daily) OR Combination therapy: Beta-lactam (Amoxicillin/Clavulanate, Cefuroxime) plus a Macrolide or Doxycycline.
• Inpatient Non-Severe: Beta-lactam (Ceftriaxone or Cefotaxime) + Macrolide OR Respiratory fluoroquinolone monotherapy.
• Inpatient Severe (ICU): Beta-lactam + Macrolide OR Beta-lactam + Respiratory fluoroquinolone.
• MRSA/Pseudomonas coverage: Add Vancomycin or Linezolid (for MRSA) and Piperacillin-Tazobactam, Cefepime, or Meropenem (for Pseudomonas) if risk factors (prior isolation or recent hospitalization with IV antibiotics) are present.

Hospital-Acquired (HAP) and Ventilator-Associated Pneumonia (VAP)

• HAP: Occurs >= 48 hours after admission.
• VAP: Occurs >= 48 hours after endotracheal intubation.
• Empiric Regimen: Must cover Pseudomonas aeruginosa and MRSA. Typically consists of an anti-pseudomonal beta-lactam (e.g., Piperacillin-Tazobactam, Cefepime, Ceftazidime, Imipenem, Meropenem) plus an anti-MRSA agent (Vancomycin or Linezolid). Two anti-pseudomonal agents are indicated if the patient has risk factors for MDR pathogens (e.g., prior IV antibiotic use within 90 days, septic shock, or acute respiratory distress syndrome).

---

2. Skin and Soft Tissue Infections (SSTIs)

• Non-Purulent Infections (Erysipelas, Cellulitis): Primarily caused by Streptococcus species. Empiric therapy targets streptococci: Oral Cephalexin, Penicillin VK, or Dicloxacillin. For IV therapy, Cefazolin or Ceftriaxone is used.
• Purulent Infections (Abscesses, Carbuncles): Primarily caused by Staphylococcus aureus (including CA-MRSA). Incision and drainage (I&D) is the primary treatment. Antibiotics are added if systemic signs are present: Oral Trimethoprim-Sulfamethoxazole (TMP-SMX), Doxycycline, or Clindamycin. For IV therapy, Vancomycin or Linezolid is used.
• Necrotizing Fasciitis: Surgical emergency requiring immediate debridement and broad IV antibiotics (e.g., Piperacillin-Tazobactam + Vancomycin + Clindamycin. Clindamycin is added for its ability to inhibit bacterial toxin production).

---

3. Bacterial Meningitis

Bacterial meningitis is an acute purulent infection of the subarachnoid space. Empiric therapy is age-dependent due to differing pathogen profiles:

• Dexamethasone Adjunctive Therapy: In adults with suspected or proven S. pneumoniae meningitis, dexamethasone (0.15 mg/kg IV Q6H) should be administered prior to or concomitantly with the first dose of antibiotics to reduce the risk of neurological sequelae (deafness, focal deficits). It should be discontinued if another pathogen is identified.

---

4. HIV Pharmacotherapy

Antiretroviral Therapy (ART) is recommended for all HIV-infected individuals. Standard initial regimens consist of two Nucleoside/Nucleotide Reverse Transcriptase Inhibitors (NRTIs) plus a third agent from another class, typically an Integrase Strand Transfer Inhibitor (INSTI).

• Preferred Initial Regimen: e.g., Bictegravir / Emtricitabine / Tenofovir alafenamide (Biktarvy).
• NRTI Toxicity Profiles:
  • Tenofovir Disoproxil Fumarate (TDF): Risk of renal toxicity (Fanconi syndrome) and bone mineral density loss. Avoid if eGFR < 60 mL/min.
  • Tenofovir Alafenamide (TAF): More stable in plasma, delivers active drug intracellularly. Reduced risk of bone and renal toxicity. Safe down to eGFR of 30 mL/min.
  • Abacavir: Requires testing for the HLA-B*5701 allele before initiation. Positive patients have a high risk of fatal hypersensitivity reactions.

---

5. Tuberculosis (TB)

Active tuberculosis is treated in two phases: an intensive phase (2 months) followed by a continuation phase (4 months, or longer in specific cases).

Active TB Regimen:
[Intensive Phase: 2 Months]
- Rifampin + Isoniazid + Pyrazinamide + Ethambutol (RIPE)
       |
       v
[Continuation Phase: 4 Months]
- Rifampin + Isoniazid (total 6 months of therapy)

RIPE Regimen Profiles and Side Effects

• Rifampin (RIF): Potent CYP3A4 inducer (causes numerous drug interactions, e.g., with oral contraceptives, warfarin, and HIV protease inhibitors). Side effects: orange-red discoloration of bodily fluids (tears, sweat, urine), hepatotoxicity.
• Isoniazid (INH): Inhibits mycolic acid synthesis. Side effects: peripheral neuropathy and hepatotoxicity. Pyridoxine (Vitamin B6) 25–50 mg daily must be co-administered to prevent peripheral neuropathy, especially in pregnant women, diabetics, alcoholics, and malnourished patients.
• Pyrazinamide (PZA): Side effects: hepatotoxicity, hyperuricemia (contraindicated in acute gout), and polyarthralgia.
• Ethambutol (EMB): Inhibits arabinosyl transferase. Side effect: optic neuritis (decreased visual acuity, loss of red-green color discrimination). Baseline and monthly visual acuity and color vision tests are required.

---

6. SFDA Guidelines & Antimicrobial Stewardship

Saudi Food and Drug Authority (SFDA) Dispensing Law

To combat the rising threat of multidrug-resistant organisms, the SFDA enforces a strict law prohibiting community pharmacies from dispensing any systemic antibiotic without a valid medical prescription signed by a licensed physician. Violation of this law leads to severe penalties, including fines up to 100,000 SAR, temporary or permanent closure of the pharmacy, and suspension of the pharmacist's professional license.

Clinical Stewardship Principles

• Pre-authorization (Restriction): Requires clinical pharmacists or infectious disease specialists to approve the use of restricted broad-spectrum antimicrobials (e.g., Meropenem, Linezolid) before dispensing.
• Prospective Audit and Feedback: Direct review of active antimicrobial therapy by the stewardship team to recommend de-escalation (switching from empiric broad-spectrum to narrow-spectrum targeted therapy based on culture results) or discontinuation.
• PK/PD Optimization:
  • Extended-Interval Aminoglycosides: Dosing aminoglycosides (Gentamicin, Tobramycin) at high doses once daily takes advantage of their concentration-dependent killing and post-antibiotic effect (PAE), while reducing the risk of nephrotoxicity and ototoxicity.
  • Prolonged Infusion Beta-Lactams: Infusing beta-lactams (e.g., Piperacillin-Tazobactam, Meropenem) over 3–4 hours instead of 30 minutes optimizes their time-dependent killing (time above MIC, $T > \text{MIC}$).