5.1 Aquation of cisplatin
After intravenous administration, the cisplatin complex dissolves in the water of the bloodstream, in which it is carried and passes into cells, crossing their membrane by passive diffusion.
What is passive diffusion?
The movement of molecules from a region of high concentration to lower concentration with no energy expenditure.
As the complex is neutral, it can easily pass through the lipophilic cell membrane.
Recent research has also suggested that the copper transporters CRT1 and CRT2 may also play a role in the uptake of cisplatin.
Is a hard or soft acid? What types of molecule or ion in the bloodstream might react with cisplatin before it gets a chance to cross the cell membrane?
There are many species present in blood, including sugars, salt, proteins, oxygen and, of course, water. is a soft acid, so soft bases pose the greatest threat, also those species that are in the greatest concentration. Thus sulfur-containing compounds, such as cysteine, might react with cisplatin, as might water.
Fortunately, in practice, the high concentration of chloride ions in the blood suppresses the hydration of cisplatin, and it passes into the cells mostly unchanged.
However, once in the cells, it is a different story.
The concentration of chloride is now much lower (4 mmol inside, compared with 100 mmol outside). Cisplatin slowly reacts stepwise with the water in the cells to form first the monosubstituted aqua complex and then the disubstituted ion.
Equation 2 shows the hydration equilibria involved.
and studies have shown that the mono-aqua square-planar complex is the active species.
This is illustrated in the next video, in which Professor Stephen Lippard (MIT) describes some of the work completed to help understand the chemistry involved.
Transcript: Video 7 The cisplatin story: Part 2. (3:11 min)
NARRATOR: No drug is perfect. Cisplatin had side effects. Apart from nausea, it could also cause kidney damage. It didn’t work against all cancers. And, most of all, there was a huge gap in understanding. How did it work? And why didn’t the trans form of the same chemical have the same effect? Better understanding could possibly lead to better drugs, and this interested top chemical research teams across the world, including a group at the Massachusetts Institute of Technology (MIT) led by Professor Stephen Lippard.
STEPHEN LIPPARD: We were primarily a chemistry group, beginning to learn something about macromolecules – proteins and nucleic acids. And we were particularly intrigued, in the case of cisplatin, with the fact that the cis isomer, with the chlorides on the same side of the square plane, was active. And the trans isomer was inactive. And I’ve always been fascinated by problems in inorganic stereochemistry. And so it seemed that there would be a structure or function relationship that we ought to be able to sort out as chemists.
NARRATOR: This is one of the tools the research chemists can use to discover details of the structure of molecules. It’s known as nuclear magnetic resonance spectroscopy, or NMR. The large shiny vessels are cryostats. They store the liquid helium which cools superconducting magnets. These magnets generate huge magnetic fields and are so heavy that NMR facilities, such as this one at MIT, are usually housed in the basement.
Test samples are lowered into the magnetic field, and radio frequency energy is fired into them. Some atomic nuclei, such as the 195 isotope of platinum, possess a property known as ‘spin’. And the combination of the two fields can make such nuclei jump from one energy state to another. This in turn produces electronic signals unique to those particular atoms, giving information on their immediate environment in the molecule.
NMR proved to be an immensely useful tool in helping to determine how cisplatin gets into the cells and what happened to it once it was inside. These chemical formulae show what the NMR results meant. Cisplatin hydrolyses stepwise to produce positively charged ions. First, one chloride ion is replaced by water to form a species with a single positive charge. And then, the second chloride ion is replaced by another water molecule to form an ion with two positive charges.
STEPHEN LIPPARD: And when that happens, the platinum becomes positively charged. The platinum complex becomes positively charged. And there would be a natural attraction then for it to bind, migrate to and bind to DNA, which is a polyanion.
How does the charge on the platinum complex change on aquation?
The complex is now positively charged.
There is a 2–3 h delay in sensitisation after the administration of cisplatin due to the slow formation of this substituted complex.
The positive charge on the substituted complex means that it is attracted to the negatively charged surface of the DNA in the cell. This was confirmed by treatment of cancer cell cultures with a high dose of -radiolabelled cisplatin, which shows where cisplatin binds in the cells.
Analyses indicated there were about 9 Pt per 1 DNA molecule, compared with ~1 Pt in protein molecules and ~1 Pt per 10–1000 RNA molecules.
In addition, it was found that there is a correlation between Pt–DNA adducts in circulating (peripheral) blood cells and disease response in patients given cisplatin.
So Pt–DNA binding has been the main focus of further studies.