Jatenzo Pediatric (Under 12) Monitoring: What Clinicians Need to Know

Why Clinicians Encounter This Question

Jatenzo is FDA-approved for hypogonadism in adult males. Full stop. The FDA prescribing information states explicitly that safety and efficacy in pediatric patients have not been established, and the product carries a warning about potential premature epiphyseal closure and virilization in children exposed to androgens [1].

Despite that, pediatric endocrinologists and urologists do occasionally evaluate pre-pubertal boys under 12 with confirmed hypogonadism, anorchia, Klinefelter syndrome, or pituitary insufficiency. When parenteral testosterone is not feasible and a compounded formulation is unavailable or declined by the family, oral testosterone undecanoate sometimes enters the conversation.

This article does not advocate for off-label use. It outlines the monitoring requirements a prescriber must implement if, in consultation with a pediatric endocrinologist and with documented informed consent, Jatenzo is used in a male under 12 years of age.

FDA Labeling and Pediatric Safety Warnings

What the Prescribing Information Says

The FDA-approved prescribing information for Jatenzo (NDA 210663) states that androgens should be used with caution in children because of possible premature epiphyseal closure and resultant short stature [1]. The label does not provide weight-based pediatric dosing tables for children under 12 because no pharmacokinetic data in that population were submitted to the FDA at approval [1].

The agency's Pediatric Research Equity Act (PREA) framework requires sponsors to study drugs in pediatric subgroups unless a waiver is granted [2]. Tolmar received a partial waiver for neonates, infants, and children under 12 based on a determination that the condition does not occur in that population with sufficient frequency, a conclusion that remains contested in the subspecialty literature for conditions such as Klinefelter syndrome and congenital anorchia [3].

Controlled Substance Scheduling

Testosterone is a Schedule III controlled substance under the Controlled Substances Act [4]. That status imposes additional documentation requirements for any off-label pediatric use, including written prescription records, no refills beyond six months, and state-level PDMP reporting in most jurisdictions [4].

Black Box Warning Relevance

Jatenzo's label carries a blood pressure warning derived from the key trial showing a mean increase in systolic blood pressure of approximately 4 to 5 mmHg at maximum dose [1]. In pediatric patients, whose baseline cardiovascular risk profiles differ substantially from adult males, that elevation warrants more frequent monitoring than the label's standard quarterly recommendation for adults [5].

Pharmacokinetics in the Pediatric Context

Absorption Depends on Fat Content

Oral testosterone undecanoate bypasses first-pass hepatic metabolism by absorbing through intestinal lymphatics as a component of chylomicrons [6]. This mechanism requires co-ingestion with a meal containing at least 19 grams of fat to achieve therapeutic serum levels [1]. In young children, meal composition and volume are highly variable, which creates pharmacokinetic unpredictability that does not affect injectable formulations to the same degree [6].

A study published in the Journal of Clinical Endocrinology and Metabolism by Swerdloff et al. (2020, N=166) demonstrated that 87% of adult males reached a serum total testosterone within the normal range (300 to 1,000 ng/dL) at three months on standard dosing [7]. No comparable pediatric pharmacokinetic data exist for oral testosterone undecanoate in the under-12 population.

Weight-Based Dosing Considerations

Adult Jatenzo titration starts at 158 mg twice daily and adjusts to 237 mg or 396 mg BID based on mid-dose serum testosterone drawn 3 to 5 hours post-dose [1]. In a child under 12 with a body weight of 20 to 35 kg, the adult starting dose may produce markedly supraphysiologic levels. Pediatric endocrinology consultation and pharmacokinetic monitoring through a laboratory capable of liquid chromatography-tandem mass spectrometry (LC-MS/MS) testosterone assays are necessary before any dose escalation [8].

LC-MS/MS is the preferred assay method because immunoassay-based testosterone measurements in prepubertal children are notoriously inaccurate at low concentrations [8]. The Endocrine Society's clinical practice guideline on testosterone therapy recommends LC-MS/MS for all measurements where clinical decisions hinge on concentrations below 150 ng/dL [9].

Core Monitoring Parameters

Serum Testosterone

Serum total testosterone should be measured at the mid-point of the dosing interval, which for twice-daily dosing means approximately 3 to 5 hours after the morning dose [1]. The FDA prescribing information specifies this sampling window explicitly because Jatenzo produces a peak-and-trough pharmacokinetic profile, unlike parenteral depot formulations [1].

Target range in pediatric off-label use is not established by guideline. The Endocrine Society's 2018 androgen deficiency guideline recommends targeting age-appropriate testosterone concentrations rather than adult normal ranges in pubertal induction [9]. In a pre-pubertal boy under 12, that likely means concentrations consistent with early Tanner stage II rather than adult male mid-normal values.

Draw serum testosterone at weeks 4, 8, and 12 following initiation, then every 3 months if levels are stable [1]. Any dose adjustment resets the 4-week recheck cycle.

Bone Age Radiography

Premature epiphyseal closure is the most serious growth-related adverse effect of androgen exposure in skeletally immature patients [10]. Bone age is assessed by plain radiograph of the left hand and wrist, interpreted using the Greulich-Pyle or Tanner-Whitehouse atlas [10].

For any pediatric patient under 12 receiving exogenous testosterone, a baseline bone age radiograph before initiation and repeat imaging every 6 months is the standard of care described in pediatric endocrinology literature [10]. If bone age advances more than 1 year ahead of chronological age in a 6-month interval, the benefit-risk ratio must be re-evaluated and dose reduction or discontinuation considered [11].

The Lawson Wilkins Pediatric Endocrine Society (now the Pediatric Endocrine Society) and the European Society for Paediatric Endocrinology jointly recommend this 6-month interval in their guidelines on androgen use in children [11].

Growth Velocity

Height and weight must be plotted on CDC growth charts at every clinical encounter, with a minimum frequency of every 3 months [12]. Velocity is calculated as centimeters per year from at least two measurements separated by 3 to 6 months using a calibrated stadiometer with the child standing without shoes.

A drop in height velocity percentile by more than 25 percentile points from baseline suggests accelerated epiphyseal maturation and warrants immediate bone age radiograph regardless of the scheduled interval [12]. Weight gain exceeding expected norms may indicate fluid retention, a known androgen side effect [1].

Hematocrit and Hemoglobin

Androgens stimulate erythropoiesis. The FDA prescribing information requires hematocrit measurement before starting Jatenzo, at 3 to 6 months, and annually thereafter in adults [1]. In pediatric patients, the monitoring interval should be shortened to every 3 months given the absence of pediatric safety data [13].

If hematocrit exceeds 54%, the prescribing information instructs clinicians to hold therapy until hematocrit returns to an acceptable level and then re-initiate at a lower dose [1]. In a growing child, erythrocytosis carries additional risk because pediatric reference ranges for hematocrit are lower than adult male ranges at baseline [13]. A value exceeding 50% in a child under 12 warrants clinical review even if it falls below the adult discontinuation threshold.

Blood Pressure

The Jatenzo prescribing information includes a dedicated blood pressure warning section, citing data from the key trial showing mean systolic blood pressure increases of approximately 3 to 5 mmHg [1]. The FDA recommends monitoring blood pressure before starting and periodically during treatment [1].

For pediatric patients, the American Academy of Pediatrics' 2017 clinical practice guideline on childhood hypertension defines elevated blood pressure using age-, sex-, and height-specific percentile tables rather than absolute values [5]. A systolic BP above the 95th percentile for age on three separate occasions meets the diagnostic threshold for hypertension in children [5]. Blood pressure should be measured at every clinic visit, using an appropriate cuff size calibrated for the child's arm circumference.

Liver Function

Oral testosterone undecanoate at the doses used in Jatenzo is not 17-alpha-alkylated and therefore does not carry the hepatotoxicity risk associated with older oral androgens such as methyltestosterone [14]. The FDA prescribing information does not require routine liver function test monitoring for Jatenzo [1]. Baseline alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are reasonable at initiation in pediatric patients, with repeat testing at 6 months and then annually [14].

Lipid Panel

Androgens alter the lipid profile, typically lowering HDL-cholesterol and potentially raising LDL-cholesterol [15]. The American Heart Association recommends lipid screening in children aged 9 to 11 years as part of universal screening [15]. Any child in this age range starting Jatenzo should have a fasting lipid panel at baseline and again at 6 months, with subsequent annual monitoring [15].

Virilization Surveillance

Physical Examination Parameters

In a pre-pubertal boy under 12, androgen exposure produces virilization signs that are expected in physiologic puberty but may occur at inappropriate ages or rates when driven by exogenous hormones [16]. Clinicians must document at each visit:

• Testicular volume using a Prader orchidometer
• Pubic hair staging (Tanner staging)
• Penile length and girth
• Axillary hair development
• Acne distribution and severity
• Voice change (subjective assessment and parental report)

Rapid virilization, defined as Tanner stage advance of more than one stage per 6-month interval, suggests supratherapeutic testosterone exposure and requires immediate dose review [16].

Psychosocial and Behavioral Monitoring

Androgens affect mood, aggression, and libido [17]. In adults, these effects are generally self-limiting and dose-related. In children under 12, the developing brain may respond differently to androgen exposure. Structured behavioral assessment using standardized tools such as the Child Behavior Checklist (CBCL) at baseline and every 6 months provides a documented record of any behavioral change [17].

Parents and caregivers should receive explicit counseling at initiation that behavioral changes, including increased irritability or aggression, may occur and should be reported promptly [17].

Dose Adjustment Protocol for Pediatric Patients

The following framework is proposed based on adult FDA-label titration logic adapted for pediatric weight and developmental considerations. No pediatric randomized controlled trial data exist to validate specific thresholds.

Step 1: Baseline assessment. Confirm diagnosis with morning serum testosterone by LC-MS/MS, bone age radiograph, LH, FSH, and karyotype if clinically indicated. Obtain baseline CBC, lipid panel, blood pressure percentile, and ALT/AST.

Step 2: Starting dose selection. For a child under 12, initiation at one-half of the adult 158 mg BID starting dose (approximately 79 mg BID, equivalent to one 158 mg capsule split or a compounded formulation) may reduce the risk of supraphysiologic exposure. Discuss with the dispensing pharmacy whether the Jatenzo capsule can be split; the current softgel formulation is not designed for splitting, so compounded oral testosterone undecanoate may be preferable in this weight range.

Step 3: Week 4 recheck. Draw serum testosterone 3 to 5 hours after the morning dose. If testosterone is below the low-normal target for intended developmental stage, increase the dose. If it exceeds the target, reduce the dose.

Step 4: Quarterly review. At months 3, 6, 9, and 12, repeat serum testosterone, CBC, and blood pressure. At months 6 and 12, add bone age radiograph, lipid panel, and ALT/AST.

Step 5: Annual benefit-risk reassessment. Document growth velocity trend, bone age advancement rate, and pubertal progression. Reassess whether oral testosterone undecanoate remains the most appropriate delivery route.

Drug Interactions Relevant to Pediatric Patients

Corticosteroids

Both androgens and corticosteroids promote fluid retention. Children with adrenal insufficiency requiring hydrocortisone replacement who also receive Jatenzo carry a combined risk of edema [1]. Clinicians should monitor weight, blood pressure, and for periorbital or pedal edema at each visit in this scenario.

Insulin and Antidiabetic Agents

Testosterone may improve insulin sensitivity, potentially lowering blood glucose in children with concurrent diabetes or insulin resistance [18]. If a child under 12 is receiving insulin alongside Jatenzo, glucose monitoring frequency should increase during the first 4 to 8 weeks of testosterone therapy, and insulin doses may require reduction [18].

Anticoagulants

Androgens potentiate the effect of warfarin by an incompletely characterized mechanism [1]. While warfarin use in children under 12 is rare, any concurrent anticoagulant therapy warrants INR monitoring within 2 weeks of starting or changing Jatenzo doses [1].

Contraindications and When to Withhold

Jatenzo is absolutely contraindicated in males with known or suspected prostate or breast carcinoma, although these malignancies are exceedingly rare in the under-12 age group [1]. The prescribing information also lists serious hypersensitivity reactions to any component as a contraindication [1].

Relative contraindications relevant to pediatric use include:

• Untreated sleep apnea (androgens worsen upper airway tone) [19]
• Hematocrit above 50% at baseline in a child under 12
• Uncontrolled hypertension above the 95th percentile for age [5]
• Active hepatic disease with ALT above 3 times the upper limit of normal [14]

Communication with Families

Informed consent in pediatric off-label use requires discussion of the following points with legal guardians:

1. Jatenzo has no FDA-approved indication for children under 12.
2. Long-term safety data in this age group do not exist.
3. Premature epiphyseal closure may result in shorter adult stature than genetically anticipated.
4. Monitoring will be frequent and includes blood draws, radiographs, and physical examinations every 3 months.
5. The decision to continue, change route, or stop therapy will be reviewed at every visit based on clinical response and monitoring results.

The Endocrine Society's 2018 clinical practice guideline on male hypogonadism states: "We suggest that clinicians inform patients about the uncertainty of long-term risks of testosterone therapy and discuss monitoring strategies" [9]. That principle applies with even greater weight in a pediatric population outside the approved indication.

Practical Monitoring Schedule at a Glance

| Parameter | Baseline | Month 1 | Month 3 | Month 6 | Month 12 | Annually | |---|---|---|---|---|---|---| | Serum testosterone (LC-MS/MS) | Yes | Yes | Yes | Yes | Yes | Yes | | CBC/Hematocrit | Yes | No | Yes | Yes | Yes | Yes | | Bone age radiograph | Yes | No | No | Yes | No | Yes | | Lipid panel | Yes | No | No | Yes | No | Yes | | ALT/AST | Yes | No | No | Yes | No | Yes | | Blood pressure (age-specific percentile) | Yes | Yes | Yes | Yes | Yes | Yes | | Growth velocity (height/weight) | Yes | No | Yes | Yes | Yes | Yes | | Tanner staging / virilization exam | Yes | No | Yes | Yes | Yes | Yes | | Behavioral assessment (CBCL) | Yes | No | No | Yes | No | Yes |