20.5: Treatment for Pulmonary Arterial Hypertension: Receptor Tyrosine Kinase Inhibitors and Calcium Channel Blockers

Receptor tyrosine kinase inhibitors (TKIs) and calcium channel blockers (CCBs) are two critical categories of drugs employed in the treatment of pulmonary artery hypertension (PAH). PAH is a disease that causes high blood pressure in the pulmonary arteries, resulting in chest pain, fatigue, and shortness of breath.

TKIs, such as imatinib (Gleevec), are particularly effective in tackling the growth and mitogenic factors that become upregulated in PAH patients. These factors contribute to the abnormal growth and proliferation of cells in the pulmonary arteries, which can lead to narrowing or blockage of these vessels. The mechanism of action of TKIs primarily involves the inhibition of platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) receptors. By blocking these receptors, TKIs can prevent the excessive cell proliferation, growth, migration, and contraction observed in pulmonary artery smooth muscle cells (PASMCs) and pulmonary vascular fibroblasts. These agents are generally administered orally. However, it's essential to note that further clinical trials are necessary before imatinib can be officially included in PAH treatment regimens, as it is not yet FDA-approved. The clinician should also be aware of potential adverse effects, such as subdural hematoma.

CCBs work differently than TKIs. They target the increasing cytoplasmic calcium levels in PASMCs. The rise in intracellular calcium levels can lead to pulmonary vasoconstriction and vascular remodeling, both of which contribute to the pathogenesis of PAH. CCBs inhibit voltage-dependent, receptor-operated, and store-operated calcium channels, lowering calcium ions' influx into the cells and preventing these harmful changes. CCB therapy typically begins with low nifedipine, amlodipine, or diltiazem doses. The dosage is then gradually increased until the maximal tolerated dose is reached. Adverse effects associated with CCBs include hypotension, hypoxemia, and potential deterioration of right ventricular function. CCBs are only effective for a subset of PAH patients.

In conclusion, both TKIs and CCBs play vital roles in managing PAH. They target different aspects of the disease process, offering a comprehensive approach to treatment. However, their use must be carefully monitored due to the potential for severe side effects. Individual treatment plans should always be tailored to the patient's specific needs and tolerances.