14 Cisplatin and other platinum containing compounds

14.1 A Brief history on the discovery of the DNA cross-linker cisplatin

In 1965, Barnet Rosenberg and colleagues discovered that platinum electrodes generated a soluble platinum complex, which inhibited fission in E. coli. At that time they were studying bacterial growth, not cancer. Fission is a form of asexual reproduction and cell division used by all prokaryotes. This process results in the reproduction of a living prokaryotic cell. Although bacterial growth continued in the presence of the platinum compound, cell division was arrested and the bacteria grew as filaments up to 300 times their normal length (Figure 14.1).

a) Normal *E. coli* and b) *E. coli* bacteria grown in the presence of a platinum-containing compound (Rosenberg 1965)

Figure 14.1: a) Normal E. coli and b) E. coli bacteria grown in the presence of a platinum-containing compound (Rosenberg 1965)

Later, a novel planar platinum complex, cisplatin, was discovered that turned out to be even more effective in inducing filamentous growth. The transplatin isomer does not exhibit a comparably useful pharmacological effect, but it is toxic and cisplatin batches are tested for the absence of the transisomer.

Comparison of cisplatin and transplatin complexes

Figure 14.2: Comparison of cisplatin and transplatin complexes

Later it was shown that cisplatin reduced the mass of sarcomas in rats. Cisplatin was approved for use in testicular and ovarian cancer in 1978. Again, it is quite an old but effective cancer drug. Although cisplatin is frequently designated as an alkylating agent, it has no alkyl group and therefore cannot carry out an alkylating reaction. It is correctly classified as an “alkylating-like” agent.

It was pure coincidence combined with scientific curiosity that the anti-tumour activity of cisplatin was discovered. The anti tumour effects were discovered in the 60th, and the drug approved in the late 70th. This is a 50 years old drug still in clinical use.

14.2 Mechanism of Action of Cisplatin: Aquation

What is the mechanism of action of cisplatin in tumour cells? Following administration to the bloodstream (Figure 12.3), cisplatin maintains a stable neutral state, because of the high concentration of cloride ions (100 mM). Inside the cell, because of the lower chloride concentration of about 4 mM, one of the chloride ligands is replaced by water in a process termed aquation, retaining the cis-configuration. This aqua ligand in the resulting complex is itself easily replaced, allowing the platinum atomto bind to bases. The preferred base is guanine, as we have also heard for alkylating agents. Crosslinking can occur via displacement of the other chloride ligand, typically by another guanine.

Schematic view of the aquation mechanism for cisplatin leading to covalent cross-linking of cisplatin to DNA and other cellular constituents (add ref).

Figure 14.3: Schematic view of the aquation mechanism for cisplatin leading to covalent cross-linking of cisplatin to DNA and other cellular constituents (add ref).

DNA binding is the main biological event that triggers anticancer properties. Formation of cisplatin adducts significantly alters the structure of the target DNA. Cisplatin can crosslink DNA in several different ways, interfering with cell division. The damaged DNA elicits DNA repair mechanisms, which in turn activate apoptosis when repair proves impossible. 1,2-intrastrands d(GpG) and d(ApG) adducts (Figure 14.4) form nearly 90% of the adducts. 1,3-intrastrand d(gpXpG) adducts also occur but are readily excised by the nucleotide excision repair (NER) system (see chapter 21). A small percentage of interstrand cross-links and monofunctional adducts are also present. As we have already learned for alkylating agents, cisplatin also predominantly binds to the N-7 position of guanine bases, because this position is the most reactive one.

Binding of ciplatin to double-stranded DNA resulting in 1,2 and 1,3 adducts. The preferred binding is through N-7 position of guanine, similar to alkylating agents.

Figure 14.4: Binding of ciplatin to double-stranded DNA resulting in 1,2 and 1,3 adducts. The preferred binding is through N-7 position of guanine, similar to alkylating agents.

14.3 Side effects of cisplatin treatment

Cisplatin binds to a variety of cellular targets, many of them source of toxicity and undesired side-effects. Consequently, the mechanism of action of cisplatin is very complex. You do not have to understand this in detail, we only want you to show how complex the mechanism is.

A variety of proteins bind to DNA, following cisplatin damage ,initiating downstream signalling pathways (Figure 14.5). One class of proteins selectively recognise DNA-cisplatin lesions, includingDNA-damage recognition proteins. Another class of targets are proteins involved in DNA packaging or DNA dependent functions such as histones and DNA or RNA polymerases. The significance of these interactions to the anti-tumour mechanism is unknown, but they are likely to at least affect the general patient toxicity profile.

Interaction of a variety of proteins with DNA, after damage by cisplatin leading to various downstream signalling pathways (Figure from Jung 2007).

Figure 14.5: Interaction of a variety of proteins with DNA, after damage by cisplatin leading to various downstream signalling pathways (Figure from Jung 2007).

As an example, we want to look at one class of proteins, the so-called high mobility group (HMG) proteins. Platinum modification distorts the structure ofdouble-stranded DNA. HMG domain proteins preferentially bind to DNA with bent or distorted structures. Not astonishingly, they therefore interact with cisplatin-modified DNA. There is very good evidence for this, since the crystal structure of the cisplastin-DNA complex bound to a HMG domain has been determined (Figure 14.6). As a non-sequence-specific DNA binding protein, it regulates numerous nuclear functions including replication, transcription, recombination and general chromatin remodelling. Despite the wealth of information, however, it cannot be stated with certainty that this DNA-binding protein domain plays an essential role in conveying the anticancer activity of cisplatin.

Crystal structure of the HMG protein –DNA complex (add ref and PDB ID)

Figure 14.6: Crystal structure of the HMG protein –DNA complex (add ref and PDB ID)

14.4 Administration, Clinical Uses and Side Effects of Cisplatin

Cisplatin is administered intravenously as a short-term fusion.

Its clinical uses are small cell lung cancer, colorectal and ovarian cancer. Cisplatin in combination with bleomycin and vinblastine is particularly effective against testicular cancer with cure rates of up to 85%.

As you can imagine, cisplatin comes with a number of side-effects /toxicities. Acute toxicity include severe nausea and vomiting. More than 90% of all patients receiving cisplatin will suffer from this side effect.

Delayed toxicities include ototoxicity, which is hearing loss that may be severe or peripheral neurotoxicity, which is nerve damage. Another side effect can be bone marrow depression, which is the decrease in the production of red and white blood cells and platelets. Nephrotoxicity, which is kidney damage, is the dose-limiting toxicity (DLT). A decreased creatinine clearance, which can be monitored using a simple blood test, indicates poor renal function. This can be prevented and managed by adequate hydration.

Bibliography

Rosenberg, B., Van Camp, L., Krigas, T. (1965). Inhibition of cell division in Escherichia coli by electrolysis products from a platinum electrode. Nature 205, 698–699.

Jung, Y. & Lippard, S.J. (2007). Direct Cellular Responses to Platinum-Induced DNA Damage. Chem. Rev. 107, 1387-1407.