Ortho-iodohippurate

Ortho-iodohippurate is a diagnostic agent used primarily in the evaluation of renal function, specifically for measuring effective renal plasma flow (ERPF). It is often referred to by its abbreviation, OIH, and is also known as Hippuran. The compound is labeled with iodine isotopes, typically either I-123 (for imaging studies) or I-131 (for older, now less common, quantitative studies).

Chemistry

Ortho-iodohippurate is an iodinated derivative of hippuric acid. The molecule consists of a benzene ring substituted with an iodine atom (ortho to the carboxylic acid group) and a nicotinamide group, forming a compound that is readily cleared by the kidneys. The presence of the iodine atom allows the compound to be labeled with radioactive isotopes of iodine, making it detectable by nuclear medicine imaging techniques.

Pharmacokinetics

After intravenous administration, ortho-iodohippurate is rapidly distributed in the extracellular fluid and is almost exclusively excreted by the kidneys. Its clearance from the bloodstream is primarily through active tubular secretion in the renal proximal tubules, with a small component of glomerular filtration. The high specificity of OIH for renal tubular secretion makes it an excellent marker for assessing ERPF.

Clinical Use

The primary application of ortho-iodohippurate is in the measurement of effective renal plasma flow (ERPF), which is a key indicator of renal function. The procedure involves the intravenous injection of OIH followed by serial measurements of its concentration in the blood or imaging of its distribution and clearance using a gamma camera. The rate of clearance from the plasma gives a direct measure of ERPF.

Indications

• Evaluation of renal perfusion and function in patients with suspected or known renal impairment.
• Assessment of renal transplant viability.
• Determination of split renal function (differential renal function between the two kidneys).

Contraindications

• Hypersensitivity to ortho-iodohippurate or any component of the formulation.
• Pregnancy, due to the potential risk of radiation exposure to the fetus.

Safety and Precautions

Patients undergoing a diagnostic procedure with ortho-iodohippurate should be well hydrated to ensure optimal renal clearance and to minimize radiation exposure. The use of radiolabeled compounds requires careful consideration of the radiation dose to the patient, and procedures should be performed in accordance with guidelines for minimizing radiation exposure.

Future Directions

With the advent of newer imaging agents and techniques, such as those based on MRI and PET, the use of ortho-iodohippurate has declined. However, it remains a valuable tool in specific clinical scenarios where its unique pharmacokinetic properties and high specificity for renal tubular function provide important diagnostic information.