cisplatin and DNA binding
/ SISS-pla-tin /
One of the most successful anticancer drugs in the world is not an elaborate organic molecule but a tiny, simple platinum complex called cisplatin. It was discovered almost by accident in the 1960s, when researchers noticed that bacteria stopped dividing near platinum electrodes, and it went on to make several once-lethal cancers, especially testicular cancer, highly curable. It is a clear example of inorganic coordination chemistry turned into medicine.
Cisplatin is a square-planar platinum(II) complex, cis-PtCl2(NH3)2 — a central platinum with two ammonia ligands and two chloride ligands arranged so the two chlorides sit next to each other (the cis arrangement, not across from each other). That geometry is everything: the trans isomer, with the chlorides opposite, is far less effective. Inside a cell, where chloride is scarce, the two chloride ligands slowly leave and are replaced by water, making the platinum reactive. The activated complex then reaches DNA and binds, the platinum forming bonds to nitrogen atoms (mostly N7 of guanine) on the same strand, usually two neighboring bases. This crosslink kinks and distorts the DNA double helix, blocking the cell's ability to copy and repair its DNA, and a cell that cannot copy its DNA cannot divide — which hits fast-dividing cancer cells hardest.
Cisplatin matters as a proof that a metal ion, placed in the right geometry, can be a precision drug, and it launched a whole field of metal-based medicines. But it comes with honest hard truths: it also damages healthy fast-dividing tissues, causing serious side effects like kidney toxicity, nausea, and nerve damage; tumors can become resistant to it; and the all-important detail is the geometry — only the cis isomer works well, a vivid lesson that in coordination chemistry, where the ligands sit relative to each other can decide whether a molecule is a life-saving drug or nearly useless. Note also that the platinum stays platinum(II) throughout; this is substitution chemistry (ligand exchange), not redox.
Inside the cell, cisplatin loses one chloride to water and the resulting Pt center binds the N7 of a guanine; it then loses the second chloride and grabs an adjacent guanine, stapling two neighboring bases together and bending the DNA.
Only the cis isomer can form this two-base crosslink; the trans isomer cannot, and is far less effective.
Geometry, not just composition, makes the drug: the trans isomer has the same atoms but is much less effective. The platinum stays Pt(II) — this is ligand substitution, not redox.