Chapter 1 — Case Study 1: The Thalidomide Story, Beginning to End
A pharmaceutical biography, told across seventy years, that illustrates why structure matters, why stereochemistry matters, and why a molecule's first story is rarely its last.
1. The drug (1953–1957)
In 1953, a small pharmaceutical company in West Germany called Chemie Grünenthal began screening organic compounds for anticonvulsant activity. One of them, synthesized by Wilhelm Kunz by reacting phthalic anhydride with $\alpha$-aminoglutarimide, was designated K-17 and given the trade name Contergan. The molecule, which we now call thalidomide, showed no useful anticonvulsant activity. But in animal studies it did appear to be a remarkably mild sedative — producing sleep without respiratory depression, apparently impossible to overdose on, and causing none of the hangover-like effects of the barbiturate sedatives then in common use.
Grünenthal brought thalidomide to market in West Germany in October 1957 as an over-the-counter sedative and anti-nausea drug. It was marketed aggressively — particularly to pregnant women suffering from morning sickness — and within three years was available in forty-six countries. (The United States was a notable exception, due to FDA reviewer Frances Oldham Kelsey, who refused to approve the drug until Grünenthal provided additional safety data that never materialized.)
At peak, thalidomide was the most prescribed sedative in Germany and had become the second-most prescribed drug of any kind, after aspirin.
2. The epidemic (1959–1961)
Beginning in late 1959, pediatricians in West Germany and Australia noticed an alarming cluster of birth defects: infants born with severe limb malformations, a condition called phocomelia (from the Greek phoca, seal, because affected children had flipper-like limbs where arms or legs would be). Some were born with no arms or legs at all. Others had internal organ malformations. Many were stillborn. The number of affected births rose rapidly through 1960 and into 1961.
In November 1961, two physicians working independently — Widukind Lenz in Hamburg and William McBride in Sydney — published the connection: mothers of affected children had taken thalidomide during the first trimester of pregnancy. The drug was withdrawn from the German market on 27 November 1961. Other countries followed over the next two years.
The best current estimate is that approximately 10,000 children were born with thalidomide-induced birth defects, and at least that many more were stillborn or miscarried. Perhaps 5,000 of the affected children survived infancy. Many are still alive today, in their sixties, and some have become prominent activists for drug-safety reform.
3. The chemistry (1961–2000)
How did a molecule that seemed to cause only benign sedation produce such devastating birth defects?
The short answer, which took decades to establish, is stereochemistry. Thalidomide has one chiral carbon — one tetrahedral carbon with four different groups attached — and therefore exists as two enantiomers, called $R$ and $S$.
When thalidomide is synthesized by Kunz's original route (or by any standard industrial synthesis), the product is a 50:50 mixture of $R$ and $S$ enantiomers. This is called a racemate or a racemic mixture, and until the 1980s it was the default way to produce and sell most drugs. The drug was tested as a racemate, regulated as a racemate, and administered as a racemate.
But the two enantiomers, it turned out, have very different biological activities:
- The $R$ enantiomer is responsible for most of the sedative activity — the effect Grünenthal originally identified.
- The $S$ enantiomer is a potent teratogen — a substance that causes developmental malformations in the embryo.
Studies in the 1990s showed that if you administer pure $R$-thalidomide to pregnant rabbits, you still get birth defects. This initially seemed to contradict the $R$/$S$ story. The resolution was discovered in 1994 by Takashi Eriksson and colleagues: thalidomide racemizes in the body. Even if you start with pure $R$, it converts to a mixture of $R$ and $S$ under physiological conditions in a matter of hours. The chiral center is alpha to two carbonyl groups and is rapidly deprotonated and reprotonated in vivo. You cannot fix the thalidomide problem by making only the $R$ enantiomer.
(This finding is important for the book: it shows that stereochemistry in solution is not the same as stereochemistry in the solid state. A drug can be enantiopure as it leaves the factory and racemize within hours inside the body. Chapter 8 returns to this in depth.)
4. The regulatory aftermath (1961–1980s)
The thalidomide tragedy permanently changed pharmaceutical regulation. The United States, which had been spared because of Kelsey's refusal, strengthened the FDA through the 1962 Kefauver-Harris Amendment, requiring pharmaceutical companies to demonstrate efficacy — not just safety — before approval. Other countries followed with similar reforms.
More specifically for chemistry, the tragedy led to rules requiring that:
- Chiral drugs must be characterized enantiomer by enantiomer during development. Both enantiomers must be tested for activity and toxicity.
- Where possible, drugs should be developed as single enantiomers. By the 2000s, the majority of newly approved drugs are single enantiomers, not racemates.
- Pregnancy is a special category in drug approval. The FDA introduced pregnancy categories (A, B, C, D, X) specifically in response to thalidomide, and virtually all drug labels today address pregnancy risk explicitly.
For the organic chemistry student, the direct pedagogical consequence is that every modern synthesis course gives stereochemistry its own substantial treatment. The fact that we have an entire Part of this book on stereochemistry (Part II) — rather than treating it as one topic in one chapter — is a direct descendant of this tragedy.
5. The rehabilitation (1964–present)
In 1964, an Israeli physician named Jacob Sheskin was working at the Hadassah Medical Center in Jerusalem when he noticed an unexpected effect. A patient suffering from erythema nodosum leprosum (ENL) — a devastating inflammatory complication of leprosy — had been given a small dose of thalidomide to help him sleep. Within hours, the patient's skin lesions began to heal. Sheskin tried it on another patient. Same result. He eventually treated more than 4,000 ENL patients over the next decades and saw dramatic improvements.
This was thalidomide's first rehabilitation. It was approved by the FDA in 1998 for the treatment of ENL, under strict protocols (pregnancy testing required before every prescription, only physicians specially registered may prescribe).
The next rehabilitation came in the 2000s, when hematologists at Johns Hopkins, Harvard, and elsewhere began testing thalidomide in multiple myeloma — a cancer of plasma cells. Thalidomide and its derivatives (lenalidomide, pomalidomide) proved to be effective treatments. Lenalidomide (Revlimid) became, for a time, the top-selling cancer drug in the world.
How do these molecules work? The answer, worked out in 2010 by Takumi Ito at Tokyo Medical University, turns out to be one of the most elegant results in modern chemical biology. Thalidomide binds to a protein called cereblon, which is part of a cellular machinery called the CRL4 ubiquitin ligase complex. This complex marks other proteins for destruction by attaching small ubiquitin tags to them. When thalidomide binds cereblon, it changes the shape of the cereblon surface — which in turn changes which proteins the CRL4 complex targets for destruction. In multiple myeloma cells, thalidomide-bound cereblon starts destroying Ikaros and Aiolos, two transcription factors the cancer depends on. The cancer cells die.
The most recent act of rehabilitation is that thalidomide has become the scaffold for an entire new class of drugs called proteolysis-targeting chimeras (PROTACs). A PROTAC is a small molecule with two halves: one half binds cereblon (often a thalidomide-derived piece), and the other half binds some disease-causing protein of interest. The molecule brings the disease protein into the proximity of cereblon, which tags it for destruction. Using this strategy, chemists have designed PROTACs that degrade estrogen receptor (for breast cancer), androgen receptor (for prostate cancer), BET proteins (for various cancers), and — most excitingly — undruggable proteins that classical small-molecule drugs cannot inhibit. PROTACs are arguably the hottest area of medicinal chemistry as this book goes to press.
And the part of every PROTAC that binds cereblon is a derivative of the very molecule that caused the phocomelia epidemic of 1961.
6. What the thalidomide story teaches the chemistry student
Several lessons, layered.
At the level of molecular reasoning: A single chiral carbon — one tetrahedral geometry, one atom — can be the difference between a useful drug and a teratogen. Nothing in your training as a chemist is more important than learning to take three-dimensional structure seriously.
At the level of mechanism: The racemization of thalidomide in vivo is itself a Chapter 27 mechanism problem. The chiral carbon of thalidomide is $\alpha$ to two carbonyl groups, which makes its proton unusually acidic ($pK_a$ around 9). A base removes the proton, producing an enolate; reprotonation can occur on either face, producing either enantiomer. The same $\alpha$-carbon chemistry that drives the aldol and Claisen reactions (Chapter 28) is what prevents us from fixing thalidomide by synthesizing only one enantiomer.
At the level of drug discovery: A molecule's biological properties are shaped by its shape, and shape can change in solution. A drug that is inert in the bottle may racemize, isomerize, or be metabolized to an active form inside the body. This is why pharmacokinetics (what the body does to the drug) is as important as pharmacodynamics (what the drug does to the body).
At the level of history: A compound's story is never over. Thalidomide in 1960 was a catastrophe. Thalidomide in 1990 was a curiosity. Thalidomide in 2020 is the backbone of a $10-billion-per-year drug class. The same molecule, understood more deeply, has become a gift to medicine.
If you learn only one thing from this case study, let it be this: do not confuse your current understanding of a molecule with the final understanding of it. A hundred years ago, the chemists of the world thought they knew what aspirin did. (They knew it relieved pain; they did not know about prostaglandins, let alone cyclooxygenase inhibition.) Fifty years ago, they thought they knew what thalidomide did. (They were wrong in all directions.) What we think we know about any molecule today is provisional. The best chemists — the ones who make the important discoveries — hold their beliefs about molecules tentatively enough to be surprised.
This book is going to give you a lot of mechanisms and a lot of reactions. It is also going to try, in every chapter, to leave you ready to be surprised.
Further reading on the thalidomide story:
- Stephens, T., and Brynner, R. (2001). Dark Remedy: The Impact of Thalidomide and Its Revival as a Vital Medicine. Perseus Publishing. A journalistic treatment, highly readable.
- Ito, T., et al. (2010). Identification of a primary target of thalidomide teratogenicity. Science, 327(5971), 1345–1350. The primary paper identifying cereblon.
- Chamberlain, P. P., and Hamann, L. G. (2019). Development of targeted protein degradation therapeutics. Nature Chemical Biology, 15(10), 937–944. Review of PROTACs, including the thalidomide connection.