Drug Discovery
- WSP Rhodes
- 1 day ago
- 14 min read
In my previous post, I touched on how modern drug discovery works and how it sometimes intersects with premodern medicines. I didn’t go into as much detail as I would have liked to as it wasn’t relevant to the topic at hand, so I decided to make that a separate post. I have gathered the stories of how several well-known pharmaceuticals were identified and turned into medicines, with the goal of showing just how diverse and complex this process can be. For all these stories, I’m going to gloss over a fair amount of detail, just sticking to the initial discovery that a certain active ingredient was effective, better than an alternative, or manufacturable. I will leave out the clinical trials and studies that demonstrated these drugs were effective and safe enough to be used as medicines, which I’ve talked about previously. Please know that trials were being done at every step for the presented drugs. I also may gloss over some of the people who performed this research, particularly if the work was done by a pharmaceutical company since that would mean there were hundreds of people involved in the drug’s development. Science is rarely done by single lone inventors, particularly in the modern era and particularly in a field as complex as pharmacology. If there is interest, I may do a part two with different well-known drugs.
Aspirin
As I mentioned in a previous post, aspirin was originally derived from a natural product. The active ingredient of aspirin is acetylsalicylic acid, a molecule that relieves pain, fever, and inflammation by downregulating a specific family of enzymes linked to these functions. Plants that contain high amounts of salicylic acid, such as willows, have been used in traditional medicine dating back 4,000 years in regions ranging from Greece to China. Chewing willow bark or drinking tea made from it was found to relieve cold, fever, rheumatism, inflammation, childbirth pain, and many others. The first experiments we might call a clinical trial for willow bark were done by Edward Stone of the Royal Society of London in 1763, confirming that powdered willow bark was effective at treating fever. Without a modern understanding of chemistry, the explanation he put forth was that fevers came from swamps, so the cure must obviously come from swamps as well. In 1828 and 1829, chemists first in Germany and then in France isolated salicylic acid from willow extract, before using it to treat rheumatism. Soon after, chemical and pharmaceutical companies such as the German company Bayer, began industrial production of salicylic acid as a pain remedy. But salicylic acid came with pretty severe side effects, particularly gastrointestinal issues such as nausea, bleeding, and ulcers (recognize those?). So in 1895, Bayer chemists Felix Hoffmann and Arthur Eichengrün* began work to chemically modify salicylic acid to lessen these side effects. By this time, Bayer had been successful at making drugs more potent by acetylating them, that is chemically modifying them by adding an acetyl group**. This made it easier for the molecules to pass through cell membranes into the bloodstream before they could strongly affect the stomach. After clinical trials, Bayer would patent acetylsalicylic acid under the name Aspirin, abbreviating Acetyl Spiraea (Spiraea being the latin name of meadowsweet, the plant from which Bayer harvested salicylic acid).
Diagram of acetyl group being added to salicylic acid
Aspirin would be an incredibly popular painkiller throughout the early 20th century. But while the chemical modification would make the side effects of the drug less severe, stomach issues were still present. This led to research into finding safer versions of the drug with fewer side effects. Salicylic acid works by attaching to and preventing the function of Cyclooxygenase (COX), a family of enzymes with multiple functions around the body. COX enzymes play roles in pain signaling, triggering inflammation, protecting the stomach lining, and clotting blood. But since COX is a family of proteins, it is possible to affect some members of the family and not others. Modern COX-affecting drugs, such as ibuprofen and acetaminophen, only affect COX-2, the enzyme responsible for pain and inflammation signaling, but doesn’t affect COX-1, the enzyme responsible for blood thinning and protecting the stomach lining. This quest for alternatives to aspirin led to discoveries about how the drug actually worked, something no one really knew until the 1970s. Today, aspirin is mostly used as a blood thinner, an effect it wasn’t found to have until the 1950s.
Insulin
I’ve very briefly talked before about Type I diabetes mellitus, but to recap, insulin is a hormone the body produces that tells certain cells (fat, muscle, and liver cells) to take sugar from the blood and repackage it in an unusable, insoluble form (glycogen). The pancreas also secretes the hormone glucagon, which tells cells to convert glycogen back into sugar. Working in concert, this keeps usable blood sugar at a consistent level. Type I diabetes occurs when the immune system incorrectly identifies insulin-producing cells as a threat to the body and kills them all off. Without the ability to produce insulin, blood sugar becomes dangerously high. There are a number of ways this leads to damage, but the most common way before the discovery of insulin was ketoacidosis. To oversimplify, having more dissolved sugar in the blood causes the bloodstream to retain water***, pulling water away from cells and tissues. Excess water in the bloodstream leads to increased urination, which ironically expels this sugar as well as other critical dissolved substances like salts and potassium. Expelling water leads to chronic dehydration and expelling sugar leads to the body converting fat into energy, which makes the blood more acidic. Between lack of energy, acidifying blood, and the loss of critical salts, diabetes was historically a horrible disease to endure. The average age of diagnosis was between 4 and 14 and even with the best pre-insulin treatment (usually starvation diets), the average life expectancy was less than two painful years.
Since antiquity, it was understood that diabetes had something to do with excess sugar, as the urine of patients tasted abnormally sweet (yes, urine tasting was a diagnostic method for much of history). In 1788, English physician Thomas Cawley published the first paper suggesting a link between diabetes and the pancreas, based on autopsies of diabetes patients. In 1889, German physicians Joseph von Mering and Oskar Minkowski confirmed this link by removing a dog’s pancreas to create symptoms identical to diabetes. Further research over the coming decades confirmed which part of the pancreas was linked to diabetes and how to extract the important sugar-metabolizing secretion. Finally, in 1921, a team from the University of Toronto, including doctors Frederick Banting and John McCloud, developed a technique to reliably extract insulin from beef pancreases and purify it to the point of usability in human patients. When the first human trials began, the effects were nothing short of miraculous, with patients recovering almost immediately after injection and life expectancy of recipients rising by over thirty years.
At first, insulin was extracted from cow and pig pancreases from slaughterhouses and purified into a usable drug. The biggest problem with this method was that it was using non-human insulin. It had the potential to cause allergic reactions, and was difficult to scale up to industrial outputs. Research into insulin would continue for decades and by the 1950s, it was understood that insulin wasn’t a mix of proteins as many had assumed, but was instead a single protein. Not only that, but it was relatively simple for a human protein. Most of our proteins go through a complex process of modification after being produced to create a final product, processes that aren’t found in simpler organisms. Insulin was a simple enough protein that bacteria could potentially produce it. By the 1980s, recombinant DNA technology was making deliberate genetic engineering possible. Since insulin was simple enough to be produced by bacteria and was an important enough drug to be worth the effort of research and development, the companies Eli Lilly and Genentech partnered to insert the gene for human insulin into a strain of E. coli bacteria. Today, virtually all pharmaceutical insulin is produced by genetically-engineering microorganisms.
Amoxicillin
I’ve touched briefly on antibiotic discovery before, but let’s go into more detail. I’ll focus on amoxicillin because of its fairly common use, but know that other antibiotics have similar-but-unique stories as well. To recap, the first antibiotic (penicillin) was discovered in 1928 by Scottish chemist Alexander Fleming. At the time, Fleming was doing research on the bacteria Stapholocaucus aureus, a clade of bacteria responsible for numerous types of infections (staph infections, if you will). This took the form of taking pus from infected patients and culturing it on petri dishes containing a growth medium (“bacteria food”) in order to grow the bacteria colony large enough to be directly observed. In August of 1928, Fleming took a vacation, leaving several petri dish cultures in his office unattended. One of these dishes had not been closed properly, so mold spores from the environment managed to get inside (mold also likes bacteria food). When Fleming returned, he found the sections of the plate with mold growing were surrounded by a region where no bacteria grew. Fleming cultured this mold and studied its secretions to find they were actively killing the bacteria in order to prevent infection and to outcompete it for food. In 1939, scientists Howard Walter Florey and Ernst Boris Chain would develop a technique to purify these secretions into an effective drug, creating a usable form of the drug penicillin.
While penicillin was a miracle drug and just in time for World War II (efforts were taken by the Allies to prevent the Axis discovering and mass producing penicillin), it wasn’t perfect. There were numerous associated side effects, it only affected gram-positive bacteria****, it couldn’t be taken orally because it couldn’t survive the stomach, and resistant bacteria began to be observed as early as 1942. So starting in the 1950s, labs around the world began making modifications to the naturally-occurring penicillin molecule and testing these modified molecules for their efficacy against gram-negative bacteria and for their acid resistance. From these experiments, the penicillin-derivative ampicillin (penicillin with its phenyl group replaced with a phenylglycine) was found by the Beechem Group and began use as a drug in 1961. While ampicillin was better than penicillin in many respects, it still had its own weaknesses. Ampicillin could be taken orally; however, only 30-40% of the drug would actually get absorbed into the blood. This made the drug less potent and any of the drug that wasn’t absorbed would cause gastroenterological side effects. So the researching process continued, this time with modifications being made to ampicillin instead of penicillin. In 1972, the same group developed amoxicillin (adding a hydroxyl group to the phenylglycine they’d just added), which increased the oral absorption to 90%. Today, amoxicillin is the most commonly prescribed antibiotic, useful against bronchitis, pneumonia, ear infections, UTIs, salmonella, Lyme disease, and gonorrhea, just to name a few.
Escitalopram
The history of psychiatric medication could be a post in and of itself, as the underlying causes of mental illnesses are often less well understood than maladies of other parts of the body. I’ll focus on escitalopram (Lexapro), but know I’m giving an overview of just a small part of this field. The first antidepressants were discovered by accident; Iponiazid was originally developed to be a treatment for tuberculosis and Imipramine was developed as an antihistamine. In clinical trials performed in the early 1950s, test subjects who happened to have severe depression were found to see improvements in their symptoms. Both drugs were found to have somewhat similar mechanisms; they increased the levels of neurotransmitters within the brain, thus strengthening the signals sent by neurons. You see, to send a signal between each other, a transmitting neuron will secrete molecules into the space between itself and a receiving neuron. These neurotransmitters attach to proteins on the surface of the receiving neuron, causing it to fire to send the signal to the next neuron in the chain. But this signal doesn’t last very long, as these neurotransmitters get reabsorbed by the transmitting neuron or get broken down by enzymes. Imipramine (and other Tricyclic antidepressants (TCAs)) block the transmitting neuron from reabsorbing neurotransmitters while Iponiazid (and other Monoamine Oxidase Inhibitors (MAOIs)) prevent neurotransmitter breakdown, meaning the neurotransmitters are able to send their signals multiple times in order to strengthen them. This effect combined with the observed mood improvement of patients led to the development of Chemical Imbalance Theory, stating that depression is caused by imbalances of these neurotransmitters.
Unfortunately, these early antidepressants caused severe side effects, ranging from cognitive impairment to severe high blood pressure when eating certain foods. Since these medications work by increasing the levels of all neurotransmitters, the next step for researchers was to figure out which specific neurotransmitters were linked to depression and to find drugs that affected them exclusively. It was found that restricting the uptake of just the neurotransmitter serotonin was effective for preventing depression symptoms. Serotonin is connected to several functions within the brain including mood, learning, sleep, pain, and many others and researchers are still trying to understand what the commonality of all these functions are. But by the early 1980s, a new family of selective serotonin uptake inhibitors (SSRIs) had begun to appear on the market, the first of which being Fluoxetine (Prozac). But a side effect of our lack of understanding of the underlying mechanism of depression is that particular drugs won’t be as effective for each patient and it’s often unpredictable which drugs will work for which patients without experimentation. This led to the development of multiple SSRI drugs to increase the likelihood that one will be effective for everyone. Since this time, the Chemical Imbalance Theory has largely fallen out of favor among psychiatrists. Today, depression is understood to be less of a single disease with a single cause and cure, but instead to be something similar to fever, a major symptom that can be caused by multiple otherwise unrelated conditions.
One of the early SSRI drugs was Citalopram, produced by the Danish company Lundbeck beginning in 1989. It had similar effectiveness and side effects as other SSRI drugs at the time. But it was understood fairly early in the drug’s development that only half of citalopram molecules were actually effective at treating depression. You see, one of the consequences of organic molecules being as complex as they are is that it is possible to have all the same atoms and functional groups arranged in different ways, creating two molecules that look deceptively similar but behave very differently. Specifically, two molecules can be built such that they are mirror reflections of each other, having the same parts but are arranged as exact opposites of each other. The best way to think about this is by looking at your hands; each hand is a perfect mirror image of the other, but it’s impossible to position your left hand such that it looks like your right hand. For these chiral molecules, two molecules can have the same mass and chemical properties, but their slightly different arrangements means that they can have very different interactions with other organic molecules. This is a big problem for drug manufacturing, as the chemical processes used to manufacture these compounds for use in pharmaceuticals will often produce an equal amount of left- and right-handed versions of the molecules. Because drugs work by interacting with other organic molecules, the two versions of the drug can have wildly different effects. A tragic example of this was the drug thalidomide; the right-handed version of the molecule was found to be an effective treatment for morning sickness, so it was given as a medication for pregnant women starting in 1957. But the manufacturing process produced an equal number of left-handed and right-handed molecules, and left-handed thalidomide inhibits the growth of blood vessels, causing birth defects in the resulting children. Citalopram was also a mixture of left- and right-handed versions of the molecule. By 1992, it was understood that only left-handed citalopram actually contributed to alleviating depression, but it wasn’t believed that right-handed citalopram was harmful. But when research later in the decade suggested that right-handed citalopram might interfere with the efficacy of left-handed citalopram, a new version of the drug was developed with only the left-handed molecule. A common way to denote the chirality of a molecule in writing is by adding an R- (for rectus, Latin for right) or S- (for sinister, Latin for left) to the beginning of the molecule. Since this new drug only used S-citalopram molecules, it was named escitalopram.
Diagram of two chiral molecules (amino acids). Note that this is meant to be a 2D image of 3D molecules; the R you see is coming toward you. Notice how it’s impossible to position either molecule such that it's identical to the other, much like your hands.
Paclitaxel
The final drug I’ll talk about is one you might be less familiar with (at least I hope you’re not), but I include it for thematic reasons. The inspiration for my previous post, and by extension this post, was a then-recent trip I took to the National Botanic Gardens in DC. One exhibit there was on Medicinal Plants, which contained the tree this drug comes from. I recognized the name from my day job in a cancer research lab, where Paclitaxel is commonly used in research. Paclitaxel is a chemotherapy drug useful against breast, ovarian, lung, and pancreatic cancer with ongoing research into its effectiveness against several others. It was first discovered as part of the Cancer Chemotherapy National Service Center (CCNSC), a major research project by the United State’s National Cancer Institute to identify compounds that could be useful for treating cancer, including natural agents. From 1960 to 1981, researchers collected roughly 30,000 samples from various plant species to test their secretions for anti-cancer properties. One such sample was the bark of the Pacific yew tree (Taxus brevifolia). Like most plants, yew trees produce toxins to prevent infection or predation. But this particular poison works by interfering with the process of cell division (specifically it interferes with how dividing cells divide up their chromosomes between the two daughter cells, ensuring both get full copies of the parent’s genome), which would make them perfect for targeting cancer cells.
It would take about ten years of clinical trials for paclitaxel to be well understood enough to be administered to patients, but there was another major obstacle to its use as a drug. To produce enough paclitaxel to treat one patient required harvesting two to ten 100-year-old yew trees, necessarily killing members of a near-threatened species. This was an obstacle just for researching the compound, so using it as a drug would be nearly impossible. So, researchers looked into paths for semi-synthesis; finding a similar molecule that is easier to source and modifying it into paclitaxel. The process that was developed involved taking the needles of the European yew tree (the species is significantly more populous than the Pacific yew tree and and needles can be harvested without harming it), extracting a compound called10-Deacetylbaccatin by treating the needles with solvents, and putting this compound through a multistep chemical process that turns it into paclitaxel. By 1995, the drug could be given to patients without any environmental risk. Along with paclitaxel, The CCNSC almost single-handedly created the cancer pharmacological industry and when asked how many drugs came out of it, then NCI Director Vince DeVita is quoted as saying “up until 1990, all of them.”
I like talking about research and development on this blog because I like to show how we know what we know. I’ve talked before about how I don’t want anyone to trust in anything blindly, but how I believe modern scientific institutions have more than earned the public’s trust. The best way I have to square that circle is to demonstrate how we know what we know and the complex history that brought us to where we are today. So I hope I’ve shown you with these five pharmaceuticals just how complex drug discovery can be, with diverse sources of inspiration, unique challenges along the way, and the hard work of numerous people. Know that every drug you use has a story similar to these.
For More Details
*The creation of aspirin is contested between these two. Eichengrün was leading the project in question with Hoffmann under his supervision, but Hoffmann argued he was the sole inventor in 1934. Given that Germany was under Nazi rule at this time and Eichengrün was Jewish, he wasn’t able to publicly refute this and it has been argued that giving sole credit to Hoffmann was politically motivated. That said, Hoffmann did own the US patent on Aspirin and Bayer continues to maintain that Hoffmann invented aspirin. To be as charitable to both men as possible, Hoffmann was the first to synthesize aspirin, but Eichengrün did the majority of the work in clinical trials and getting the drug approved.
**A brief bit of organic chemistry, acetyl groups consist of two carbon atoms, one of which is double-bonded to an oxygen atom (see diagram above). It is an example of a functional group; a particular arrangement of carbon, hydrogen, and other atoms that give a complex organic molecule particular chemical properties. During this time period, scientists determined the structures of organic compounds using a multi-stage process; reacting them with multiple chemicals to determine what functional groups were present, burning them to determine the number of atoms in each molecule, and using the known properties of different elements within these compounds to reverse engineer the structure from the data collected. As you’ll see, a lot of drug research consists of taking known molecules, replacing bits of them with other functional groups, and testing them to see if there’s a different effect.
***This is due to osmosis. In short, having a large amount of dissolved solute (sugar or otherwise) makes it harder for water molecules to move around within the solution. This means it is far easier for water molecules outside the bloodstream to enter it (there’s less sugar blocking their movement) then it is for water molecules inside the bloodstream to exit it. Since more water molecules are entering the blood than exiting, water over time concentrates in the bloodstream.
****Gram-positive and gram-negative are the two most common clades of bacteria. Gram-positive bacteria have thicker cell walls (protective shells) while gram-negative bacteria have a thinner cell wall that is protected by a layer of lipids. Gram-positive bacteria include staphylococcus and streptococcus while gram-negative bacteria include E. coli, chlamydia, and Y. pestis.
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