DTPA has high stability constants for many metals. That doesn't necessarily mean that it will bind these metals in the in vivo environment, but at least it means that it is possible. The explanation is that many of these metals, for example Mercury (Hg) may exist in a number of species in vivo, and these will also have varying stability constants.
Here are some log stability constants of DTPA for various metals (the higher the number, the stronger the metal binding):
Metal Log K
Thus, DTPA would be expected to be good for mercury (Hg) removal, but as mentioned above, it would also depend how stable the species Hg is in, in that individual. If we look at Lead (Pb), or cobalt (Co), note that the stability is extremely similar to Zn. However Ca-DTPA has a much lower stability (10.74) than Pb-DTPA (18.80) or Co-DTPA (18.40), so Ca-DTPA would be effective at removing these metals, but would also remove Zn. If used alone, Ca-DTPA would lower Zn in blood (hypozincemia) because the binding constants for Pb, Co and Zn are so close. One of the rationales of our standard chelation approach of Ca-DTPA on day 1 and Zn-DTPA on day 2, the Zn-DTPA on day 2 would not only chelate heavy metals but also correct the Zn decrease.
It may not be possible to determine in what speciation any of these metals are in, in any one individual.
What to do?
The most logical is to obtain 24 hr urine for metals prechelation, and then 24 hr urine starting immediately after injection with Ca-DTPA on day 1, and collect 24 hr urine just before administration of Zn-DTPA. This would inform if DTPA (or any other chelator) is able to remove the metal in question (increased urine content of that metal on postchelation would indicate removal).
It is also logical to administer Zn-DTPA on day 2 to not only restore Zn, but also continue to chelate heavy metals. Also of value, after 2 days post chelation to take multivitamins (or sports drinks) that include other native metals like Mg, Mn, Cu, and Fe.
Richard Semelka, MD.