Platelet and Tissue cAMP: Novel Biomarkers of Milrinone Efficacy in Children

Milrinone is a phosphodiesterase type III (PDE3) inhibitor with sites of action in cardiac
and vascular smooth muscle. PDE3 hydrolyzes the critical second messenger cyclic adenosine
monophosphate (cAMP), and PDE3 inhibition (PDE3i) results in a positive inotropic effect in
the heart through an increase in cAMP. PDE3i causes relaxation of the vascular smooth muscle
and induces vasodilation while concomitantly reducing myocardial oxygen consumption.

In adults, long term PDE3i results in malignant arrhythmias and an increased risk of sudden
death, and is therefore not a recommended therapy in this population. However, in the
pediatric Heart Failure (HF) population, long-term outpatient milrinone infusions are safely
used as a palliative therapy or as a bridge to transplant without an increased risk of
unexpected deaths, and results in fewer HF emergency department visits and admissions and
improved New York Heart Association (NYHA)/Ross classification. In addition, short term PDE3i
(i.e. milrinone) is successfully and routinely used in children presenting with decompensated
HF following cardiac surgery. In children undergoing cardiac surgery with cardiopulmonary
bypass there is a predictable fall in cardiac index 6 to 18 hours after cardiac surgery. This
phenomenon is known as low cardiac output syndrome (LCOS) and is typified by tachycardia and
poor perfusion resulting in end-organ dysfunction and risk for cardiac arrest. Milrinone is
the only proven drug for prophylactic use in children following cardiac surgery for the
prevention of LCOS.

While standard dosing of milrinone is routinely used in children, the actual dose-response
relationship is unknown. Indeed, the investigators have noted significant variability in
serum milrinone concentrations between patients despite weight based dosing strategies. This
uncertainty in dosing is confounded by differences in patient age, size and ontologic
maturation of the kidneys. Milrinone dose adjustments, therefore, vary widely among
practitioners based on urine output, degree of systemic vasodilation and a change in serum
creatinine. Additional uncertainty resides in the inability to easily identify the most
appropriate milrinone dose to produce a biological effect (such as an increase in the
critical secondary messenger, (cAMP) in the pediatric population.

Because milrinone is excreted as unchanged drug in the urine, kidney function is a critical
factor in milrinone dosing. Recently published data demonstrate that 73% of milrinone levels
in children with acute kidney injury (AKI) were outside the therapeutic range. Thus, children
are particularly vulnerable to inappropriate milrinone dosing. Currently, AKI is diagnosed by
an increase in serum creatinine (SCr). Unfortunately, the increase in SCr may not occur until
3 days after AKI occurs. Thus, in children treated with milrinone, undetected AKI would
result in significant over-dosing. Since milrinone is also a potent vasodilator, excess
milrinone dosing in AKI could lead to hypotension - and further exacerbation of AKI.
Therefore, early detection of AKI is especially important in children receiving milrinone.
Preliminary data demonstrate that supra therapeutic milrinone concentrations and urinary AKI
biomarkers increase in advance of SCr in patients with AKI. In this grant, the investigators
propose to determine if increases in Tissue inhibitor metalloproteinase and insulin like
growth factor binding protein-7 (TIMP2xIGFBP7) (prior to an increase in SCr) will correlate
with increased platelet cAMP and supra-therapeutic milrinone levels.

In the absence of the ability to identify the optimal dose, children are at risk for
clinically relevant over or under-dosing with milrinone that can lead to hemodynamic
compromise and end organ dysfunction. While clinical response to milrinone remains the most
important factor in dose titration decisions, in order to fully optimize milrinone dosing and
minimize drug-related toxicity, a biomarker representative of biologic milrinone effect is
needed. The Investigators recently demonstrated that milrinone treatment results in increased
myocardial cAMP levels and augmented phospholamban phosphorylation in children, but not in
adults, with idiopathic dilated cardiomyopathy. In addition, preliminary data suggest that
platelet cAMP levels correlate with tissue levels. The purpose of this study is to determine
if platelet cAMP levels can serve as a circulating biomarker for end organ (cardiac)
milrinone efficacy, and investigate whether changes in this biomarker correlate with clinical
efficacy. Defining platelet cAMP levels in pediatric patients with heart disease would
provide the basis for a personalized approach to milrinone dose titration and allow
identification of those most likely to benefit from its use.

Inclusion Criteria:

- Children aged birth to 18 year of age

- Surgical complexity STAT score > 3

- Use of cardiopulmonary bypass for cardiac surgery

- Use of milrinone intra-operatively and post-operatively

Exclusion Criteria:

- Patients on milrinone therapy prior to surgery

- Gestational age less than 34 weeks at the time of surgery

- Weight less than 2500 grams at the time of surgery

- Abnormal renal function prior to cardiac surgery. Abnormal renal function is defined
as a serum creatinine > 0.3mg/dL above baseline (lowest value in the preceding 3
months prior to surgery) compared to the level obtained immediately prior to cardiac
surgery.