7.2 Pediatric and Neonatal Pharmacotherapy

Pediatric Developmental Stages and Pharmacokinetics

Pediatric pharmacotherapy requires distinct considerations from adult medicine because infants and children are continually maturing anatomically and physiologically. The World Health Organization (WHO) and Canadian Paediatric Society (CPS) classify pediatric subpopulations as follows:

• Preterm Neonates: Born before $37\text{ weeks}$ gestational age
• Full-Term Neonates: $0\text{ to }28\text{ days}$ of life
• Infants: $1\text{ month to }12\text{ months}$
• Children: $1\text{ year to }12\text{ years}$
• Adolescents: $12\text{ years to }18\text{ years}$

+-------------------------------------------------------------------------+
|          DEVELOPMENTAL PHARMACOKINETIC (PK) MATURATION MATRIX           |
+-------------------------------------------------------------------------+
|  ABSORPTION:                                                            |
|  - Gastric pH > 4-5 at birth (reaches adult levels by age 2)            |
|    -> Increased absorption of acid-labile drugs (ampicillin, penicillin)|
|    -> Decreased absorption of weak acids (phenytoin, phenobarbital)     |
|  - Thin stratum corneum & high BSA/weight ratio -> dramatic percutaneous|
|    systemic absorption of topical steroids and toxic substances         |
|                                                                         |
|  DISTRIBUTION:                                                          |
|  - Preterm TBW ~80-85%, Term TBW ~75%, Adults ~60%                      |
|    -> Larger Vd per kg for water-soluble drugs (gentamicin, vancomycin) |
|    -> Requires HIGHER mg/kg initial / loading doses                     |
|  - Low serum albumin & reduced binding affinity -> higher free drug     |
|  - Bilirubin displacement by sulfonamides/ceftriaxone -> KERNICTERUS    |
|                                                                         |
|  METABOLISM:                                                            |
|  - Severe UGT (glucuronidation) deficiency in neonates -> toxic drug    |
|    accumulation (Chloramphenicol -> Gray Baby Syndrome)                 |
|  - CYP450 ontogeny (CYP3A7 fetal -> CYP3A4 matures over 1-2 years)      |
|  - Ages 1-9: Hepatic clearance per kg EXCEEDS adult rates               |
|                                                                         |
|  EXCRETION:                                                             |
|  - GFR & tubular secretion 30-40% of adult at birth (matures by 6-12 mo)|
|  - Prolonged elimination half-life of renally cleared drugs             |
|  - Requires EXTENDED dosing intervals (e.g., Gentamicin q24-36h)        |
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Absorption

• Gastric Acid Secretion: Gastric pH is nearly neutral ($\text{pH } 6-8$) at birth and remains $> 4-5$ throughout the first several weeks of life due to immature parietal cell function, reaching adult acidity levels by age 2. This hypochlorhydria increases the oral bioavailability of acid-labile medications (e.g., ampicillin, penicillin G) while decreasing the bioavailability of weakly acidic medications (e.g., phenobarbital, phenytoin) that require an acidic stomach for non-ionized dissolution.
• Intestinal Motility: Gastric emptying is slow and irregular in neonates and young infants, leading to delayed absorption peaks for many oral medications.
• Percutaneous Absorption: Neonates and infants possess a significantly thinner stratum corneum and a remarkably high body surface area to body weight ratio ($BSA / \text{Weight}$). Topical medications (e.g., corticosteroids, hexachlorophene, rubbing alcohol, lidocaine) are absorbed rapidly and extensively into systemic circulation, risking severe systemic toxicity and adrenal axis suppression.

Distribution

• Total Body Water (TBW) and Extracellular Fluid (ECF): TBW comprises $80-85%$ of body weight in preterm neonates and $75%$ in full-term neonates (compared to $55-60%$ in adults). Consequently, hydrophilic drugs (such as aminoglycosides [gentamicin], vancomycin, and beta-lactams) distribute into a much larger volume of distribution per kilogram. To achieve adequate therapeutic peak serum concentrations ($C_{\max}$), neonates require a higher milligram per kilogram ($\text{mg/kg}$) loading dose (e.g., gentamicin $4-5\text{ mg/kg}$ in neonates vs $2\text{ mg/kg}$ in adults).
• Protein Binding and Bilirubin Displacement: Neonates have lower circulating concentrations of total plasma proteins (albumin and $\alpha_1$-acid glycoprotein) with reduced binding affinity. Highly protein-bound drugs exhibit higher free active fractions. Critically, drugs that compete for bilirubin binding sites on albumin (e.g., ceftriaxone, sulfamethoxazole/trimethoprim) displace unconjugated bilirubin, allowing free bilirubin to cross the immature blood-brain barrier and deposit in the basal ganglia, precipitating life-threatening kernicterus (bilirubin encephalopathy).

Metabolism

Hepatic drug-metabolizing enzymes mature asynchronously (ontogeny). In neonates, Phase I CYP450 enzymes operate at low activity levels (fetal CYP3A7 predominates at birth and is gradually replaced by adult CYP3A4 over the first 1-2 years). Phase II conjugation pathways, particularly uridine diphosphate glucuronosyltransferase (UGT), are profoundly immature in neonates.

• Gray Baby Syndrome: Neonatal inability to glucuronidate chloramphenicol leads to toxic accumulation, mitochondrial respiratory chain inhibition, progressive circulatory collapse, cyanosis, hypothermia, ashen-gray skin pallor, and fatal cardiovascular collapse.
• Hypermetabolism in Young Children (Ages 1-9 Years): By age 1 to 2 years, hepatic CYP450 enzyme capacity and hepatic blood flow per kilogram exceed adult levels. Children in this age group frequently clear drugs (e.g., theophylline, carbamazepine, valproic acid, levetiracetam) faster than adults, requiring larger $\text{mg/kg}$ daily doses and more frequent dosing intervals.

Excretion

Glomerular filtration rate (GFR) and renal tubular secretion are low at birth (approximately $30-40%$ of adult values normalized to BSA in term neonates, and even lower in preterm neonates) due to incomplete nephrogenesis and low renal perfusion pressure. GFR matures rapidly during the first 3 months and reaches adult values by 6 to 12 months of age. Renally eliminated drugs (e.g., gentamicin, vancomycin, ampicillin) have significantly prolonged elimination half-lives in neonates, requiring extended dosing intervals (e.g., gentamicin administered every $24\text{ to }36\text{ hours}$ in neonates rather than every $8\text{ hours}$ as in older children).

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Pediatric Dosing Methodologies and Clinical Calculations

1. Weight-Based Dosing ($\text{mg/kg}$): The standard dosing methodology in pediatrics. Doses are specified as $\text{mg/kg/dose}$ (e.g., acetaminophen $10-15\text{ mg/kg/dose}$) or $\text{mg/kg/day}$ divided into multiple doses (e.g., amoxicillin $80-90\text{ mg/kg/day}$ divided BID).
2. Body Surface Area (BSA) Dosing ($\text{mg/m}^2$): Highly accurate method correlating with cardiac output and GFR, standard for antineoplastic agents and narrow therapeutic index drugs. Calculated using the Mosteller Formula: $\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}$
3. Maximum Adult Dose Rule: A fundamental pediatric safety principle states that a calculated weight-based pediatric dose must never exceed the maximum recommended adult single or daily dose (e.g., if a $60\text{ kg}$ adolescent is prescribed amoxicillin $90\text{ mg/kg/day} = 5400\text{ mg/day}$, the dose must be capped at the standard adult maximum of $2000-3000\text{ mg/day}$ depending on the clinical indication).
4. Pediatric Renal Function Estimation: The Cockcroft-Gault equation is not valid in pediatrics. Renal function is estimated using the Updated Bedside Schwartz Equation: $\text{eGFR (mL/min}/1.73\text{ m}^2\text{)} = \frac{36.5 \times \text{Height (cm)}}{\text{Serum Creatinine (}\mu\text{mol/L)}}$ (Note: When Serum Creatinine is measured in $\text{mg/dL}$, the formula is $\text{eGFR} = \frac{0.413 \times \text{Height (cm)}}{\text{SCr (mg/dL)}}$.)

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Toxic Excipients and Absolute Drug Contraindications

Lethal Excipients in Neonates and Infants

Pediatric Drug Contraindications Matrix

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|               ABSOLUTE PEDIATRIC DRUG CONTRAINDICATIONS                 |
+-------------------------------------------------------------------------+
|  1. CEFTRIAXONE IN NEONATES (Age < 28 days):                            |
|     - Displaces bilirubin from albumin -> Kernicterus                   |
|     - Precipitates with IV Calcium (TPN, Ringer's) -> fatal crystalline |
|       deposits in lungs and kidneys. PREFERRED AGENT: CEFOTAXIME        |
|                                                                         |
|  2. CODEINE & TRAMADOL (Age < 12 years; < 18 post-adenotonsillectomy):  |
|     - Ultra-rapid CYP2D6 metabolizers convert prodrug to massive toxic  |
|       morphine/M1 levels -> fatal respiratory arrest & death            |
|                                                                         |
|  3. ACETYLSALICYLIC ACID (ASA) in Viral Illness (Age < 18 years):       |
|     - Triggers REYE'S SYNDROME (mitochondrial injury -> acute hepatic   |
|       steatosis, severe encephalopathy, hyperammonemia, brain herniation|
|                                                                         |
|  4. TETRACYCLINES (Doxycycline, Minocycline) (Age < 8 years):           |
|     - Chelates calcium in growing teeth/bone -> permanent yellow-brown  |
|       tooth discoloration, enamel hypoplasia, temporary bone growth halt|
|                                                                         |
|  5. FLUOROQUINOLONES (Ciprofloxacin, Levofloxacin):                     |
|     - Cartilage toxicity, arthropathy, tendon rupture in growing joints |
|     - Reserved strictly for CF pseudomonal exacerbations or anthrax     |
|                                                                         |
|  6. PROMETHAZINE (Age < 2 years):                                       |
|     - Severe, fatal respiratory depression                              |
|                                                                         |
|  7. VALPROIC ACID (Age < 2 years):                                      |
|     - High risk of fatal mitochondrial hepatotoxicity & microvesicular  |
|       steatosis (POLG mutation risk)                                    |
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Canadian Paediatric Guidelines for Common Childhood Conditions

Acute Otitis Media (AOM)

• Diagnostic Criteria: Acute onset of signs/symptoms of middle ear inflammation accompanied by middle ear effusion (bulging tympanic membrane).
• First-Line Pharmacotherapy (CPS Guidelines): High-Dose Amoxicillin ($80-90\text{ mg/kg/day}$ PO divided BID) for 10 days in children $< 2\text{ years}$, or 5 days in children $\ge 2\text{ years}$ with uncomplicated disease. High doses are required to overcome intermediate penicillin-resistant Streptococcus pneumoniae via altered penicillin-binding proteins (PBPs).
• Treatment Failure or Concurrent Purulent Conjunctivitis: Switch to High-Dose Amoxicillin-Clavulanate ($80-90\text{ mg/kg/day}$ amoxicillin component with $6.4\text{ mg/kg/day}$ clavulanate in a $14:1$ or $7:1$ ratio to cover beta-lactamase-producing Haemophilus influenzae and Moraxella catarrhalis while minimizing clavulanate-induced diarrhea).

Croup (Laryngotracheobronchitis)

• Pathophysiology: Parainfluenza viral infection causing subglottic laryngeal edema and stridor.
• Treatment: Single dose of Dexamethasone ($0.6\text{ mg/kg}$ PO, IM, or IV; maximum $10-16\text{ mg}$) for all severities (mild, moderate, severe) to reduce airway inflammation and hospital admission rates. For moderate-to-severe croup with resting stridor and retractions, administer Nebulized Racemic Epinephrine ($2.25%$, $0.5\text{ mL}$ in $2.5\text{ mL}$ NS) or $L\text{-epinephrine}$ ($1:1000, 5\text{ mL}$) for rapid microvascular mucosal vasoconstriction, followed by a minimum of 2 to 3 hours of clinical observation for rebound stridor.

Pediatric Analgesia and Antipyresis

• Acetaminophen: $10-15\text{ mg/kg/dose}$ PO every $4-6\text{ hours}$ (maximum $75\text{ mg/kg/day}$ in infants/children, not to exceed $4000\text{ mg/day}$). Weight-based dosing must always be used rather than age-based dosing charts.
• Ibuprofen: $5-10\text{ mg/kg/dose}$ PO every $6-8\text{ hours}$ (maximum $40\text{ mg/kg/day}$, not to exceed $2400\text{ mg/day}$). Indicated in infants aged $\ge 6\text{ months}$. Avoid in dehydrated children or those with acute renal impairment.