Technology; Inside Human Cells, a New Frontier
By LAWRENCE M. FISHER
Published: January 24, 1993
MOST drug discovery has focused in recent years on the development of molecules that either interact with or block receptors -- proteins that act as on-off switches for genetic activity -- on the surfaces of human cells.
Now, a young biopharmaceutical company has developed a technology that targets receptors inside the cell, opening a new and compelling avenue for drug discovery.
Ligand Pharmaceuticals Inc., based in San Diego, was founded in 1988 by Dr. Ronald Evans, a scientist at the Salk Institute who identified and cloned the family of proteins that have come to be known as intracellular receptors. Ligand, which went public in November, has developed proprietary technology to screen molecules for their potential interaction with these receptors and is engaged in the development of drugs for the treatment of cancer, gynecological diseases and skin diseases.
A molecule that binds with a receptor, like a key in a lock, is called a ligand. Ligand, the company, uses chemistry to create molecules that mimic the action of naturally occurring hormones, like sex steroids, adrenal steroids, thyroid hormone and retinoids. Small and fatty, these molecules pass naturally through cell membranes and bind with the intracellular receptors to form a ligand-receptor complex. The complexes then enter the cell nucleus, where they bind with DNA, the genetic material that regulates the cell's activity. The DNA produces messenger RNA, which prompts the output of new proteins.
One validation of Ligand's approach is the existence of a number of commonly used drugs, first identified by random screening, that accomplish therapeutic effects by linking with the receptors. The drugs -- including tamoxifen, an anti-estrogen drug used to prevent recurrence of breast cancer, and Retin-A, used to treat acne and sun-damaged skin -- seem to work in much the same way that Ligand hopes its products will.
BECAUSE the different intracellular receptors are similar in shape, drugs like tamoxifen and Retin-A activate more than one at a time, prompting undesirable side effects. But Ligand's scientists believe that advances in molecular biology, coupled with their propietary screening technology, will enable them to create drugs with more specific action and fewer side effects.
"The key to drug discovery is to get as close as you can to the specific molecular interactions that are behind any given disease," said Cynthia Robbins-Roth, editor of Bioventure View, a biotechnology industry newsletter published in San Mateo, Calif. "What goes on inside the cell holds the key to the specificity of clinical actions. Ligand has a proprietary position in a critical set of molecular targets that very few other groups have access to."
Ligand's drugs will be small molecules, which means they could be given orally. And they can be manufactured with traditional chemistry, the company said, so they are likely to cost less to produce and be more stable in the body. The large proteins developed and used by biotechnology companies must be injected, because a human digestive tract would break them down if taken orally.
"This is the way I think the most mileage can be gotten out of the molecular biology revolution," said Robert B. Stein, ex-head of pharmacology at the Merck, Sharp & Dohme Research Laboratories who is now Ligand's vice president for research. "The best way to exploit molecular biology is to marry it with traditional medicinal chemistry."
Proteins can be either the causative or the curative agents of disease, so some Ligand molecules will seek to turn on genetic activity inside cells, and others to turn it off. Agonists, which cause things to happen, bind to target receptors, inducing shape changes similar to those caused by natural hormones, and also prompting DNA activity. Antagonists, which prevent things from happening, also bind but induce a different shape that blocks genetic activity.
The intracellular receptors are controlled by small molecules, making them suitable targets for small molecule drugs that mimic their natural models.
"What Ligand does better than anyone else in the world is to look at the details of the genetic activation process, and to sense more accurately and profoundly its activation or interruption," said Dr. James E. Darnelle, Vincent Astor Professor of Molecular Cell Biology at Rockefeller University in New York.
In screening numerous molecules as drug candidates, Ligand has chosen not to do animal testing, which it found too costly and slow and sometimes misrepresentative of results in humans, or biochemical assays, which it decided were fast but inconclusive.
Instead it relies on a proprietary cell-culture-based test, also originally developed by Dr. Evans at the Salk Institute, which is fast and uses the actual human receptors that are the drug targets.
Using biotechnology, Ligand replaces the normal DNA in the cells used for its test with DNA that is encoded for the protein that makes fireflies produce light. When a test molecule binds with an intracellular receptor and then prompts the DNA, sensitive instruments can then measure the amount of light produced. Ligand has automated this process so that it can screen thousands of compounds per week. Promising candidates then undergo tests in cell culture and animals.
Ligand is developing some drug candidates to sell itself and others to sell with corporate partners. It hopes to enter human clinical trials with its first compound, for the treatment of various leukemias and skin cancers, in the second half of this year. Government approval will take several years, even if things go well. The company has retained rights for drugs that are typically used by specialists, who can be serviced by a small sales force, and has chosen to collaborate on drugs with broader markets.
Ligand has a collaboration with Glaxo Holdings P.L.C. of Britain to develop drugs for the treatment of cardiovascular and lipid diseases, particularly atherosclerosis and high blood fat content. It is collaborating with Allergan Inc., of Irvine, Calif., on retinoid drugs for various cancers and skin diseases, and with Pfizer Inc., of New York City, on intracellular receptor-based drugs for osteoporosis. Ligand also has an agreement with E. Merck Darmstadt, a German pharmaceutical company, to gain access to libraries of steroids and steroid-like compounds for drug screening.
DESPITE its promising technology and various collaborations, Ligand found it difficult to go public last year without adding an unusual "sweetener" to its offering -- a promise of additional shares without charge, provided by insiders, if the stock has not gained 20 percent in the next two years. The sweetener was necessary, Ligand's management said, because so many biotechnology stocks spent much of 1992 hitting the bottom. Analysts said another factor was that Ligand chose to go public at a very young age, with no drugs in trials.
The stock was offered in November at $11 a share, raising more than $40 million. It quickly rose to $14.50 in over-the-counter trading, then fell, thanks in large part to the depressing effect of Centocor Inc. on the whole bio-stock sector (page 11), to the $9 range.
"We told investors, 'Don't invest if you don't have a strong belief in the fundamentals,' " said David E. Robinson, Ligand's president and chief executive.
Diagram/Chart: "Targeting Specific Functions of a Cell," illustrates and describes scientists use synthetic compounds called ligands to study specific cell structures. (Source: Ligand Corporation)