Treatment of Pandrug-Resistant (PDR) Infections

Definition & Context

• MDR (Multidrug-Resistant): resistant to ≥3 antibiotic classes
• XDR (Extensively Drug-Resistant): susceptible to only 1-2 classes
• PDR (Pandrug-Resistant): resistant to ALL available antimicrobial agents

General Principles

1. Source control is mandatory - drain abscesses, debride necrotic tissue, remove infected foreign bodies/catheters. This is often required for cure regardless of antibiotic choice.
2. Culture before treating - obtain cultures from the infection site and blood before initiating antibiotics.
3. Use combination therapy - with PDR organisms, combining agents (even with limited susceptibility) improves the probability that at least one agent will be active.
4. Assess clinical response - in vitro testing may not always detect resistance; clinical monitoring is essential.
5. Infection control measures - isolation, dedicated personnel, gowns and gloves to prevent spread (especially for PDR P. aeruginosa and Acinetobacter).

Drug Options for PDR/Near-PDR Gram-Negative Infections

Novel Beta-Lactam/Beta-Lactamase Inhibitor (BL/BLI) Combinations

Key: Ceftazidime-avibactam is the broadest, covering KPC and OXA-48 carbapenemases, but does NOT cover metallo-beta-lactamases (MBL/NDM).

Siderophore Cephalosporin - The Broadest Option

• Cefiderocol - Active against ESBL, KPC, OXA-48, MBL (NDM/VIM/IMP), CRPA, CRAB, and Stenotrophomonas maltophilia. This is the only agent with activity across ALL carbapenemase types. Use with caution outside UTIs and HABP/VABP due to limited clinical data.

"Last Resort" Agents

Nephrotoxicity warning: Treatment with polymyxins, especially combined with other nephrotoxic agents, carries a HIGH risk of renal toxicity - monitor renal function closely.

By Organism

PDR/CRAB (Carbapenem-Resistant Acinetobacter baumannii)

• Preferred: Sulbactam-durlobactam (novel BLI with intrinsic activity against Acinetobacter)
• Alternative: Cefiderocol
• Last resort: Polymyxin B or colistin + carbapenem combination (even when resistant, high-dose carbapenem may provide synergy) ± tigecycline ± fosfomycin

PDR Pseudomonas aeruginosa (CRPA)

• Preferred: Ceftolozane-tazobactam, ceftazidime-avibactam, or imipenem-relebactam
• Alternative: Cefiderocol
• Combination: anti-pseudomonal beta-lactam + aminoglycoside (amikacin preferred) or ciprofloxacin
• Last resort: Colistin/Polymyxin B

PDR CRE (Carbapenem-Resistant Enterobacterales)

• KPC-producing: Ceftazidime-avibactam OR meropenem-vaborbactam OR imipenem-relebactam
• MBL-producing (NDM, VIM, IMP): Ceftazidime-avibactam + aztreonam, OR cefiderocol
• OXA-48: Ceftazidime-avibactam OR cefiderocol

PDR Stenotrophomonas maltophilia

• First-line: Cefiderocol
• Alternative: TMP-SMX (if susceptible), minocycline

Site-Specific Treatment

• Pneumonia (HABP/VABP): BL/BLI combination as the beta-lactam backbone + consider aminoglycoside or fluoroquinolone if resistant risk factors. Add vancomycin or linezolid if MRSA risk.
• Bacteremia: Treat aggressively with IV combination therapy; source control paramount.
• Intra-abdominal infection: BL/BLI (e.g., ceftazidime-avibactam or ceftolozane-tazobactam) + metronidazole for anaerobic coverage; 7-10 days duration.
• UTI: Ceftazidime-avibactam, meropenem-vaborbactam, or cefiderocol (most clinical data exist here).
• Osteomyelitis: Extended 6-week course; surgical drainage if neurological compromise, instability, or treatment failure.

Emerging/Investigational Strategies

• Bacteriophage therapy - lytic phages targeting specific resistant strains; used compassionately in some PDR cases
• Nanoparticle-based antibiotics - still investigational
• Antibiotic recycling - fosfomycin and rifampin as adjuncts in combination regimens

Key Takeaways

1. PDR infections leave few or no ideal options - combination therapy is the rule.
2. Cefiderocol is the broadest single agent covering all carbapenemase types.
3. Ceftazidime-avibactam is preferred for KPC/OXA-48-producing CRE.
4. Colistin/Polymyxin B remain last-resort salvage agents but carry significant nephrotoxicity.
5. Antimicrobial stewardship and infection control are the best tools to prevent PDR emergence and spread.

• Harrison's Principles of Internal Medicine 22E (2025), Treatment of GNB Infections, p. 1326
• The Washington Manual of Medical Therapeutics, Activity of Novel Gram-Negative Antibiotics vs. Resistant Organisms
• Goldman-Cecil Medicine, P. aeruginosa chapter - Infection Control
• Harrison's (Fishman's Pulmonary), HAP/VAP Empirical Antibiotic Treatment Table

Pneumocystis jirovecii Infection (PCP)

Overview

Epidemiology & Risk Factors

• Still the most common AIDS-defining opportunistic infection in the US
• ~95% of adult cases occur when CD4+ count < 200 cells/μL (or CD4% < 14%)
• Other risk factors: prior PCP, oropharyngeal candidiasis, HIV RNA detectable despite ART

• Prolonged high-dose corticosteroids
• Chemotherapy recipients
• Solid organ transplant (especially renal transplant)
• Hematopoietic stem cell transplant (HSCT)
• Autoimmune diseases on immunosuppressants
• Hematologic malignancies

Clinical Presentation

Diagnosis

Imaging

• Chest X-ray: Bilateral, symmetric, perihilar interstitial infiltrates progressing to diffuse alveolar pattern (classic "ground-glass" appearance)
• HRCT chest: Ground-glass opacities - more sensitive when CXR is normal (present in 0-39% of PCP cases!)
• Nodules with/without cavitation, miliary pattern occasionally seen
• Adenopathy and pleural effusions are rare - suggest alternative diagnosis

Lab Markers (Supportive, Not Diagnostic)

• LDH: Elevated - non-specific but useful; very high LDH suggests more severe disease
• 1,3-β-D-glucan: Sensitive peripheral marker - elevated in PCP; helps distinguish infection from colonization when PCR is positive

Microbiologic Diagnosis (Required for Definitive Dx)

Gold standard: BAL + immunofluorescence or PCR. In HIV-negative patients, BAL silver stain alone has only 46% sensitivity - always add PCR/IFA.

Treatment

FIRST-LINE: Trimethoprim-Sulfamethoxazole (TMP-SMX)

• Rash (can treat through mild rash by reducing dose by half or desensitizing)
• Fever, nausea, vomiting
• Elevated aminotransferases
• Hyperkalemia
• Bone marrow suppression (anemia, neutropenia)
• Rare: Stevens-Johnson syndrome, toxic epidermal necrolysis, septic shock-like syndrome

Note: Patients who develop PCP despite TMP-SMX prophylaxis can usually still be treated with TMP-SMX.

ALTERNATIVE REGIMENS (for TMP-SMX allergy/intolerance)

Severity guide - Mild: PaO2 >70 mmHg or A-a gradient <35; Moderate: PaO2 50-70 mmHg; Severe: PaO2 <70 mmHg or A-a gradient >35.

ADJUNCTIVE CORTICOSTEROIDS (Critical for Severe Disease)

Adjunctive steroids also possibly helpful in HIV-negative patients with severe PCP, though evidence is less robust.

IRIS (Immune Reconstitution Inflammatory Syndrome)

• HIV+ patients starting both PCP treatment and ART simultaneously may develop a paradoxical worsening (fever, worsening infiltrates) - rarely respiratory failure
• IRIS is a diagnosis of exclusion; rarely requires stopping ART

Mechanical Ventilation (Severe/ICU)

• Acute respiratory failure from PCP carries ~30-50% mortality even with mechanical ventilation
• Use lung-protective ventilation strategies (same as ARDS)
• ART may be beneficial even in ICU setting

Prophylaxis

Primary Prophylaxis (Prevention before first episode)

• HIV+ adults/adolescents with CD4 < 200 cells/μL or CD4% < 14%
• Non-HIV: prolonged immunosuppression (solid organ transplant, HSCT, long-term steroids, etc.)

Prophylaxis Regimens

Stopping Prophylaxis (HIV+)

• May discontinue primary prophylaxis when CD4 rises > 200 cells/μL for at least 3 months on ART
• Consider stopping at CD4 100-200 cells/μL if HIV RNA undetectable for 3-6 months

Secondary Prophylaxis (after first episode)

• Same agents as primary
• Discontinue when CD4 > 200 cells/μL for ≥3 months on ART
• Caution with aerosolized pentamidine for secondary prophylaxis (higher breakthrough risk)

Recent Evidence Update (2024-2025)

G6PD Deficiency and PCP Treatment

The Core Mechanism

• NADPH production fails under oxidant challenge
• Glutathione is depleted
• Oxidative damage goes unchecked
• Hemoglobin is oxidized to methemoglobin and forms Heinz bodies
• RBCs are destroyed (hemolytic anemia)

Drugs Affected by G6PD Status

1. Dapsone - AVOID / Use with Extreme Caution

• Treatment: Dapsone 100 mg/day + TMP (alternative to TMP-SMX)
• Prophylaxis: Dapsone alone, or dapsone + pyrimethamine + leucovorin

1. Generates reactive oxygen species (ROS)
2. Oxidizes oxyhemoglobin → methemoglobin
3. Creates Heinz bodies (denatured hemoglobin visible on peripheral smear)
4. Labels RBCs for splenic destruction → hemolytic anemia

Mandatory pre-screening rule: G6PD deficiency must be ruled out BEFORE starting dapsone. This is listed as a required pre-therapy laboratory test alongside CBC and reticulocyte count.

2. Primaquine - AVOID

• Treatment: Clindamycin + primaquine (oral base 15-30 mg/day)

Rule: Screen for G6PD deficiency before prescribing primaquine. If G6PD deficient, this regimen is contraindicated.

Treatment Decision Framework by G6PD Status

Key practical point:

• TMP-SMX remains the drug of choice regardless of G6PD status - it does not carry oxidant risk to RBCs
• In a G6PD-deficient patient intolerant of TMP-SMX with mild-moderate PCP: atovaquone is the go-to alternative
• In a G6PD-deficient patient with severe PCP who cannot tolerate TMP-SMX: IV pentamidine is the preferred salvage (but carries its own toxicities - nephrotoxicity, arrhythmias, hypotension)

Prophylaxis in G6PD Deficiency

Monitoring if Dapsone is Unavoidable

• CBC + reticulocytes: weekly for month 1, then twice monthly for months 2-3
• A hemoglobin drop of 1-2 g/dL is anticipated and tolerable; more than that warrants dose reduction or drug switch
• Monitor reticulocyte count to gauge compensatory erythropoiesis
• If reticulocyte count is markedly elevated, the bone marrow is at maximum compensation - further dose increases are not safe
• Methemoglobin levels: only check if patient is symptomatic (dyspnea, cyanosis)
• Patients should carry a medication card alerting emergency providers to the hemolytic/methemoglobin-generating potential of the drug

Timing caveat: G6PD assay should be obtained when the patient is well, not immediately after a hemolytic episode - newly released reticulocytes have higher G6PD activity, which can give a falsely normal result right after hemolysis.

Treatment of Thyroid Storm (Thyrotoxic Crisis)

Overview

Treatment - The "5-Step" Framework

Correct Sequence:

STEP 1 - Block Peripheral Adrenergic Effects (Give FIRST)

Caution: Atrial fibrillation is often refractory to rate control until antithyroid therapy is started. In patients with heart failure, use diuretics + ACE inhibitors alongside.

STEP 2 - Inhibit Thyroid Hormone Synthesis (Give BEFORE Iodine)

Why PTU over methimazole in storm? PTU has the dual action of blocking synthesis AND inhibiting peripheral T4→T3 conversion via type 1 deiodinase (Dio1). Methimazole has longer duration but lacks this T4→T3 blocking effect. PTU is generally preferred in acute thyroid storm for this reason.

Methimazole is available IV outside the US, but not recommended by current American Thyroid Association guidelines.

STEP 3 - Block Thyroid Hormone Release (Give ≥1 HOUR After Thionamide)

Critical rule: Iodine given BEFORE thionamides can precipitate or worsen thyroid storm by flooding the gland with substrate for synthesis. Always wait at least 1 hour after the thionamide dose.

STEP 4 - Inhibit T4→T3 Conversion + Treat Relative Adrenal Insufficiency

When corticosteroids are combined with PTU and iodide, T3 levels can normalize within 48 hours.

STEP 5 - Identify and Treat the Precipitating Event

Empirical antibiotics if infection is suspected as precipitant. Up to 25% of cases have no identifiable precipitant.

SUPPORTIVE MEASURES (Simultaneous with Above)

ADJUNCT / REFRACTORY THERAPIES

Radioactive iodine and surgery are contraindicated during active thyroid storm - they can precipitate further crisis. Only considered after sustained euthyroid state is established.

SPECIAL SITUATIONS

• PTU preferred in first trimester (methimazole is teratogenic in T1 - aplasia cutis risk)
• Methimazole preferred in second and third trimesters (PTU carries hepatotoxicity risk)
• Beta-blockers: use cautiously; propranolol acceptable short-term; esmolol can cause fetal bradycardia
• Avoid radioactive iodine (absolute contraindication)

• Type 1 (excess iodine stimulation): Stop amiodarone + thionamide therapy
• Type 2 (destructive thyroiditis): Corticosteroids; amiodarone cessation case-by-case
• Lithium preferred over iodine (iodine-induced storm)

Drugs to AVOID in Thyroid Storm

Thyroid Storm (Thyrotoxic Crisis)

Definition

• Untreated mortality: approaches 100%
• With prompt treatment: mortality still 10-30% (up to 50% in cardiovascular presentations)
• Death occurs from arrhythmias, congestive heart failure, respiratory failure, DIC, hypoxic brain injury, or sepsis

Pathophysiology

1. Adrenergic hyperactivity - either from increased release of thyroid hormones or increased receptor sensitivity to catecholamines
2. Higher levels of free thyroid hormones - circulating T4 and T3 (bound to protein) are freed by precipitating factors; free fraction rises disproportionately
3. Excess T3 effect at the cellular level - T4 is converted to active T3, which enters cell nuclei, binds thyroid hormone-responsive elements, and drives hypermetabolism in every tissue
4. Precipitant-mediated amplification - stress, infection, trauma, or surgery frees hormones from binding sites and increases receptor sensitivity, multiplying the effect

Key: The total thyroid hormone level in storm may not be higher than in uncomplicated hyperthyroidism - it is the free fraction and tissue sensitivity that distinguish the two states.

Precipitating Factors

Clinical Features

1. Thermoregulatory

• Marked fever: 104°-106°F (40°-41°C) - often the first sign of impending crisis in a known hyperthyroid patient
• Profuse sweating

2. Cardiovascular

• Extreme tachycardia - characteristically out of proportion to the degree of fever
• Atrial fibrillation in 10-35% of cases
• Widened pulse pressure (high cardiac output + reduced peripheral vascular resistance)
• Elevated systolic BP (due to direct inotropic/chronotropic effects of T3 on the heart)
• Congestive heart failure, pulmonary edema, bibasal crepitations, pedal edema
• Hypotension and cardiovascular collapse in severe cases
• Takotsubo (stress) cardiomyopathy can occur as a manifestation

3. Central Nervous System

• Agitation, anxiety, irritability (early)
• Delirium, acute psychosis
• Tremor, weakness
• Seizures
• Coma (late/terminal)

4. Gastrointestinal-Hepatic

• Nausea, vomiting, diarrhea
• Abdominal pain
• Jaundice (hepatic failure with cholestatic jaundice - less common but poor prognostic sign)

• Goiter, exophthalmos/ophthalmopathy (if Graves' disease)
• Lid lag, stare
• Tremor, hyperreflexia
• Warm, moist skin

Diagnosis

Clinical Diagnosis Tool: Burch-Wartofsky Point Scale (BWPS)

• ≥45: Thyroid storm - treat immediately
• 25-44: Impending storm - treat aggressively
• <25: Unlikely to be thyroid storm

The BWPS is sensitive but not specific. A score ≥45 is more sensitive than the Japanese Thyroid Association criteria (TS1/TS2 classification) for detecting clinical thyroid storm.

Laboratory Findings

• TSH: Suppressed/undetectable (<0.01 μU/mL)
• Free T4 and Free T3: Elevated (free fractions more elevated than in uncomplicated hyperthyroidism)
• Total T3/T4: Unreliable - do not use (affected by thyroid-binding globulin changes)
• Hyperglycemia (increased glycogenolysis + catecholamine antagonism of insulin)
• Mild hypercalcemia (hormone-mediated bone resorption)
• Elevated LFTs, leukocytosis, mild anemia, low cholesterol

Differential Diagnosis

Why It Is an Emergency

• Cardiovascular collapse
• Respiratory failure
• Hepatic failure (jaundice = poor prognosis)
• DIC
• Coma and death