Pentetic acid

Pentetic acid or diethylenetriaminepentaacetic acid (DTPA) is an aminopolycarboxylic acid consisting of a diethylenetriamine backbone with five carboxymethyl groups. The molecule can be viewed as an expanded version of EDTA and is used similarly. It is a white solid with limited solubility in water. The conjugate base of DTPA has a high affinity for metal cations. Thus, the penta-anion DTPA5− is potentially an octadentate ligand assuming that each nitrogen centre and each –COO− group counts as a centre for coordination. The formation constants for its complexes are about 100 greater than those for EDTA. As a chelating agent, DTPA wraps around a metal ion by forming up to eight bonds. Its complexes can also have an extra water molecule that coordinates the metal ion. Transition metals, however, usually form less than eight coordination bonds. So, after forming a complex with a metal, DTPA still has the ability to bind to other reagents, as is shown by its derivative pendetide. For example, in its complex with copper(II), DTPA binds in a hexadentate manner utilizing the three amine centres and three of the five carboxylates. Like the more common EDTA, DTPA is predominantly used as chelating agent for complexing and sequestering metal ions. DTPA has been considered for treatment of radioactive materials such as plutonium, americium, and other actinides. In theory, these complexes are more apt to be eliminated in urine. It is normally administered as the calcium or zinc salt (Ca or Zn-DTPA), since these ions are readily displaced by more highly charged cations and mainly to avoid to depleting them in the organism. DTPA forms complexes with thorium(IV), uranium(IV), neptunium(IV), and cerium(III/IV). In August, 2004 the US Food and Drug Administration (USFDA) determined zinc-DTPA and calcium-DTPA to be safe and effective for treatment of those who have breathed in or otherwise been contaminated internally by plutonium, americium, or curium.