A PK Primer
How Your Body Processes the Drugs You Take
When it comes to developing new HIV medications -- and understanding how best to use those we already have available -- everyone is familiar with the importance of clinical trials. These studies provide us with important data about a drug's (or combination of drugs) side effects and its impact on viral load and CD4 counts, both in people new to HIV drug treatment and those with drug-resistant virus. But just as it's important to know how these drugs affect the body, it's also important to understand how the body affects these drugs. This is where the study of pharmacokinetics, or PK, comes in.
Knowledge about PK isn't only for pharmacologists (experts who study how chemical substances act in living systems, including the human body). Over the past several years, researchers, healthcare providers, and people living with HIV have come to appreciate that a more detailed understanding of pharmacokinetics can dramatically improve the way we use available drugs, as well as drugs that are being developed. The details surrounding what happens to a drug, from the time it enters the body to the time it's eliminated, is both fascinating and complex, and the study of the various processes involved is continuing to help researchers figure out how to use HIV drugs with better effectiveness, less toxicity, and greater simplicity.
Pharmacology and PK
As is true with virtually all medical terms, the word "pharmacology" is Greek in origin: pharmacon means drug and logos means science. The study of pharmaceuticals, or drugs, for medicinal use in people is called clinical pharmacology.
A drug has two central properties: its pharmacokinetics and its pharmacodynamics. Pharmacokinetics (PK) involves the relationship between the dose of the drug and the concentration (amount) of the drug in the body. Pharmacodynamics (PD) involves the relationship between the concentration of the drug in the body and the response it produces. In other words, PK describes what the body does to the drug, while PD describes what the drug does to the body.
In the past, PK studies of HIV drugs were simply an initial step in the development process. They were conducted in test tubes, animals, and a few people to help determine correct dosing, so that clinical trials evaluating a drug's effectiveness (for example, its effect on CD4 counts and viral load) and side effects could be conducted. Today, PK studies are much more involved. They are often conducted throughout the drug development process, and are sometimes required after the drug has been approved.
There are many reasons for the growing involvement of, and interest in, pharmacokinetics. For starters, HIV therapy involves combinations of drugs that sometimes must be used in conjunction with medications for AIDS-related complications and other diseases. This can lead to complex drug-drug interactions and it is very important for HIV-positive people and their healthcare providers to be aware of how drugs for HIV and other conditions affect each other.
Second, a drug's PK can vary considerably and depend on a number of biological factors. These include pregnancy, age, gender, race, body weight, genetics, and other illnesses (for example, liver or kidney disease). Knowing how drug concentrations vary under these circumstances is crucial, given that they can all apply to HIV-positive people.
Third, researchers now understand that HIV reproduces in many tissues throughout the body, including the brain. As a result, researchers need to determine the concentrations of HIV drugs not only in the blood, but also in the tissues where HIV replicates to ensure the drugs' effectiveness.
Fourth, understanding the PK of both new and older HIV drugs has allowed researchers to simplify treatment (for example, fewer pills or fewer doses), maximize the effectiveness of treatment, and reduce the risk or severity of side effects.
All About ADME
Every drug's PK profile depends on four factors: its absorption, distribution, metabolism, and elimination (ADME). This is a multi-step process that begins immediately after the drug, or combination of drugs, is swallowed and ends when the last trace of it leaves the body. Through all of these steps, a drug faces numerous challenges, underscoring the important role of PK research. Not only are PK studies helpful in figuring out what these challenges are, but they can help determine the best ways to avoid these challenges or use them to the patient's advantage.
In order for any drug to work, it needs to find its way into the body through a process known as absorption. This is actually a two-step process. First there is the release of the drug from its dosage form. Most HIV medications are taken orally (by mouth) in the form of a tablet, capsule, or liquid. The release of the drug from its dosage form usually takes place in the stomach. Capsules, for example, are made up of a gelatin outer coat, with the active ingredient contained inside. This allows for the drug to be swallowed and deposited in the gut, where the gelatin is dissolved by stomach acid, releasing the active ingredient.
Once the drug has been released from its dosage form, it must be moved from the gut into the portal vein, which carries the drug to the liver. With the help of stomach acid, a drug's active ingredient is made into a solution that can be absorbed by the membranes of the small intestine and then passed into the bloodstream.
The release of the drug from its dosage form, along with the movement of the drug from the gut into the portal vein, is a sensitive process. Some medications will break down too quickly -- and won't be properly absorbed -- if they come into contact with stomach acid. Videx, a nucleoside reverse transcriptase inhibitor (NRTI), is a prime example of this. Videx (didanosine) tablets contain an antacid buffer to neutralize stomach acid. Videx EC capsules use an acid-resistant gelatin coat to prevent the active ingredient from being damaged by stomach acid before it has a chance to be absorbed.
The fusion inhibitor Fuzeon (enfuvirtide) is so sensitive to stomach acid that it can't be taken orally. It must be injected under the skin using either a hypodermic needle or the new needle-free device currently being evaluated in studies.
Some HIV medications, including most of the protease inhibitors (PIs) -- especially Reyataz (atazanavir) -- require acid in the stomach in order to be dissolved and absorbed properly. This is why HIV-positive people must be very careful if they also take medications that neutralize or decrease acid production in the stomach, such as those used for heartburn or acid reflux disease. (See Table 2 in "Gastrointestinal Problems and HIV" in this issue.)
Food can also affect the absorption of HIV drugs. Videx tablets, Videx EC capsules, and the protease inhibitor Crixivan (indinavir) -- when it's used without Norvir (ritonavir) -- should be taken on an empty stomach to ensure proper absorption, whereas the PIs Viracept (nelfinavir), Invirase (saquinavir), Kaletra (lopinavir/ritonavir), Aptivus (tipranavir), Reyataz, and the NRTI Viread (tenofovir) are best absorbed when taken with food. This is one of the many basic and important issues evaluated in PK studies. Your healthcare provider or pharmacist can tell you which medications should or shouldn't be taken with food.
After the drug has been absorbed and initially processed (metabolized) -- an important step explained in the next section -- the drug re-enters the bloodstream and is rapidly distributed throughout the body. The ultimate goal during the distribution phase is to get the HIV drug to where it's needed -- inside CD4 cells. Once inside these cells, the drugs can go to work. However, there are a number of barriers that stand in the way of a drug being properly distributed throughout the body.
Protein binding is one such barrier. A drug that re-enters the bloodstream after leaving the liver will often attach to proteins in the blood, including albumin and alpha-1-acid glycoprotein. When bound to these proteins, the drug can't enter cells and is usually eliminated from the body. However, a percentage of the drug manages to evade protein binding and continues to circulate freely throughout the body. This free-circulating drug is sometimes referred to as the "free fraction."
A drug's free fraction remains constant in the circulation and is available to move into cells where it can act. Some drugs, such as PIs and the non-nucleoside reverse transcriptase inhibitor (NNRTI) Sustiva (efavirenz), are highly protein-bound, and only small fractions are protein-free. This can affect dosing; the more highly protein-bound a drug is, the higher the drug's concentration may need to be in the bloodstream to ensure that enough free fraction is available to do what it's supposed to do.
A drug's distribution also involves its concentration inside cells. With most HIV drugs, it's actually the intracellular concentration that matters most. In fact, PK studies evaluating intracellular concentrations have played a major role in simplifying treatment. For example, when the NRTIs Epivir (lamivudine) and Ziagen (abacavir) were first approved, they had to be taken twice a day, given that their levels in the blood were significantly reduced within 12 hours. Once researchers began looking at their intracellular concentrations, however, it was determined that enough drug was present inside cells over a 24-hour period to allow for once-daily dosing.
HIV doesn't only reproduce in CD4 cells in the bloodstream. It also reproduces in cells in the central nervous system (the brain), the genital tract, and the lymphoid tissues (the spleen, lymph nodes, and gut). In order for an HIV drug to stop the virus from reproducing in these compartments, it must be able to reach them. However, this can be a challenge for many drugs.
The central nervous system, for example, is protected by a network of blood vessels that make up the "blood-brain barrier." The blood-brain barrier (BBB) is semi-permeable -- it only allows some materials to cross into the brain and spinal column. The smallest blood vessels, called capillaries, are lined with endothelial cells. Between these endothelial cells are small spaces that allow substances to move between the inside and the outside of the blood vessel. In the brain and spinal column, however, the endothelial cells fit tightly together, and substances can't filter out of the bloodstream. Small drug molecules do the best job of penetrating the blood brain barrier; large molecules -- including those bound to protein -- are kept out.
Transporter proteins, including P-glycoprotein, are yet another barrier. These proteins work like pumps, flushing potentially harmful chemicals out of cells and sensitive tissues in the body. They help keep toxins out of the brain (along with the BBB), as well as the testes and the lining of the gut. And while these proteins can be credited with keeping our cells healthy, they can also be blamed for keeping potentially useful medications -- including the protease inhibitors and various chemotherapies for cancer -- away from the cells and tissues where they are most needed.
PK studies have allowed us to determine which HIV drugs are able to pass the BBB and transporter proteins into the brain. This has been very useful, especially for people suffering from HIV-related dementia, a disease likely caused by HIV reproduction and cell destruction in the brain. The NRTIs Retrovir (zidovudine) and Ziagen, and the NNRTI Viramune (nevirapine), seem to maintain the best drug concentrations in the central nervous system. The PIs, unfortunately, are less likely to achieve adequate concentrations in the central nervous system because of the challenges they face with the BBB and transporter proteins.
Pretty much everything we put into our bodies, including nutrients and medications, needs to be eliminated properly. While some HIV drugs, such as the NRTIs, are easily eliminated from the body, the NNRTIs and PIs need to be chemically broken down first. This process, known as metabolism, primarily takes place in the liver, but can also occur during the absorption stage in the gut. Because the gut and liver don't recognize drugs as serving a natural purpose in the body, their primary goal is to eliminate them. In turn, PK studies are necessary to determine how a drug is metabolized and how much active drug remains in the body after it has been prepared for elimination.
There are two types of chemical reactions, or phases, involved in drug metabolism -- phase I reactions and phase II reactions. During phase I reactions, a drug is broken down in the gut and liver, most commonly through a process known as oxidation. During phase II reactions, the drug undergoes a process called conjugation, whereby it is made more water or fat soluble (dissolvable in water or fat). This allows the drug to be excreted from the body in either urine or feces.
Phase I reactions rely on a family of proteins known as the cytochrome P450 enzyme system. In people, there are approximately two dozen types of cytochrome enzymes, eight of which are responsible for metabolizing nearly all medications. Aside from their role in metabolizing drugs, these enzymes are also responsible for metabolizing nutrients, environmental toxins, and various other substances. To further complicate the issue, certain biological changes, such as hormonal changes during pregnancy, may contribute to changes in cytochrome enzyme activities. Genetics and other diseases, such as hepatitis C, can also alter the production and activities of these enzymes. In turn, they can affect drug metabolism.
The most notable enzyme is cytochrome P450 3A4 (CYP 3A4). This enzyme affects the metabolism of all of the PIs and NNRTIs on some level. The NRTIs are exempt from phase I reactions and, as a result, don't have to deal with these enzymes.
Almost all of the PIs and NNRTIs are substrates of CYP 3A4, meaning that they rely on this enzyme to be metabolized. A few of the HIV drugs are also inducers of CYP 3A4, meaning that they have the ability to increase the activity of this enzyme. This can increase the metabolism of drugs that are substrates of CYP 3A4, therefore decreasing their concentrations in the body. Many HIV drugs, including most of the PIs, are inhibitors of CYP 3A4, meaning that they can decrease activity of this enzyme. This can slow the metabolism of drugs that are substrates of CYP 3A4, therefore increasing their concentrations in the body.
Things only get more confusing when multiple drugs are taken at the same time, especially when so many of them interact with CYP 3A4. For example, the PI Norvir (ritonavir) is a very powerful CYP 3A4 inhibitor. Once it enters the liver, it drastically reduces the activity of CYP 3A4. In turn, the metabolism of drugs that depend on this enzyme slows down dramatically. With no place to go, these other drugs begin circulating through the bloodstream and into the tissues, waiting to be metabolized.
This can be a very serious issue and underscores the importance of PK studies, especially in HIV research. When a drug is first developed, the dose is based, in part, on how much drug reaches the bloodstream after being metabolized, with no other drugs interfering with the process. If it's combined with another substrate for the same CYP enzyme, or with a drug that inhibits (or induces) the CYP enzymes it uses, its concentration in the blood may increase (or decrease).
Sometimes, this decrease or increase isn't significant enough to cause problems. Other times, however, the decrease may be significant enough to make the drug ineffective, possibly leading to the development of resistance, or the increase may be significant enough to cause serious side effects. Because of this, PK studies -- evaluating the effects one drug has on another drug's concentrations -- are an absolute necessity.
Examples of potentially harmful drug-drug interactions abound, especially with Norvir. Many sedatives (Halcion and Versed, for example) are substrates for CYP 3A4, which is inhibited by Norvir. As a result, blood levels of these sedatives can become dangerously high, potentially resulting in coma or death if they're taken with Norvir. Similarly, Norvir's inhibitory effect on CYP 3A4 can result in high levels of many cholesterol-lowering drugs, most notably Zocor and Mevacor. This too can have serious consequences. While these sedatives and cholesterol-lowering drugs shouldn't be used with Norvir, PK studies have helped researchers to determine the best alternatives -- and the best doses to use -- to avoid problems.
PK research helps determine the best dosing for HIV medications. The key is to keep drug levels above the minimum effective concentration (MEC) -- and well above the inhibitory concentration 50 (IC50) -- to prevent HIV from replicating. The minimum concentration (Cmin) should never be allowed to go below the MEC. It is also important to keep the maximum concentration (Cmax) of the drug below the minimum toxic concentration (MTC). The green line represents the best drug dose, as the Cmin is above the IC50 and MEC and the Cmax is below the level associated with unnecessary side effects.
PK studies have also allowed researchers to take advantage of drug-drug interactions. For example, the PIs Norvir and Crixivan are both substrates and inhibitors of CYP 3A4. Norvir is a more potent CYP 3A4 inhibitor than Crixivan and it causes Crixivan levels to increase in the bloodstream when the two are taken together. Without potent inhibition of CYP 3A4, two Crixivan capsules need to be taken three times a day on an empty stomach to ensure proper drug levels.
Researchers took what they knew about Norvir and Crixivan and conducted PK studies. As expected, a low dose of Norvir slowed down the metabolism of Crixivan and increased its concentration in the bloodstream significantly. In turn, a number of dosing improvements were possible. The total daily dose of Crixivan could be reduced from six capsules to four capsules; the number of times it needed to be taken each day was decreased from three to two times a day; and it did away with the need for Crixivan to be taken on an empty stomach.
Similar tactics have been used for almost all of the other PIs, including Reyataz and Invirase. In fact, Norvir is considered a necessity when taking Crixivan, Invirase, and the most recently approved PI, Aptivus.
After drugs are metabolized and distributed, they must be eliminated. For some drugs, metabolism in the gut or liver takes care of this, either by making the drug water soluble (for elimination in urine) or fat soluble (for excretion in bile and elimination in feces). Some drugs, including the NRTIs, are eliminated by the kidneys.
Just as liver health is very important to the proper elimination of drugs via the liver, kidney health is very important to the proper elimination of drugs via the kidneys. Kidney problems or disease can impair the kidneys' ability to eliminate drugs in the urine. This can cause concentrations of the drug that depend on kidney elimination to increase in the blood. Similarly, certain drugs -- such as Benemid (probenecid), a treatment for gout -- can prevent other drugs, including Retrovir, from reaching kidney cells and slow down their elimination by the kidneys.
Putting It All Together
Knowing that a drug faces many challenges during the absorption, distribution, metabolism, and elimination stages of its time in the body, researchers conduct PK studies to ensure two central factors:
That enough drug is administered to effectively treat the disease, and
That the drug dose is low enough to avoid or reduce the risk of side effects.
This, however, is easier said than done, and a great deal of research is often needed to make sure that the dose is just right for everyone taking the drug, especially when drug interactions are possible.
It all begins with the establishment of the IC50 -- the inhibitory concentration (50). This represents the minimum amount of drug needed to reduce HIV replication by 50%. Research also sets out to determine a drug's minimum effective concentration (MEC), the drug concentration that must be maintained in the blood to ensure effectiveness. The MEC is usually set much higher than the IC50. Researchers can then make estimates as to how much drug needs to be maintained in the bloodstream -- and inside cells -- for HIV medications to remain active against the virus.
In PK studies involving animals and both HIV-negative and positive people, researchers first set out to determine a drug's bioavailability. This refers to the amount of drug that reaches the blood after it has been administered and initially processed in the gut and the liver (this initial processing is known as first-pass metabolism). Bioavailability is usually expressed as a percentage. Drugs administered intravenously (through an IV line) are likely to have a bioavailability of 100%, whereas drugs administered by mouth -- which must first be absorbed and undergo first-pass metabolism -- are likely to have a lower percentage. If the percentage is too low, a higher dose may be necessary.
To increase a drug's bioavailability, researchers may also explore different formulations of their medications. For example, the PI Lexiva is an improved version of Agenerase. Both drugs contain the same active ingredient, amprenavir. Lexiva (fosamprenavir) is a prodrug of amprenavir, meaning that it is converted to the active ingredient once it has passed into the bloodstream. Studies have demonstrated that this improves the bioavailability of amprenavir, resulting in fewer gastrointestinal side effects and higher blood concentrations requiring many fewer pills a day.
After determining a drug's bioavailability, the drug enters more advanced PK studies, involving HIV-negative and/or HIV-positive people who check into a research unit for several days. Blood is collected from these study volunteers frequently -- usually every hour -- to monitor drug levels over time, either after a single dose or after several doses.
One PK characteristic researchers look for is the maximum concentration, the Cmax for short. A drug's maximum concentration in the blood is achieved after absorption and first-pass metabolism. By comparing maximum concentrations and side effects that occur in these studies, researchers work to find the best tolerated maximum dose. The concentration at which unnecessary side effects start to occur is sometimes referred to as the minimum toxic concentration (MTC). The time it takes for an administered drug to achieve its maximum concentration is referred to as the Tmax.
After the maximum concentration is achieved, the amount of drug begins to decrease. The lowest concentration in the bloodstream, before a second dose is taken and increases the concentration of the drug again, is known as the minimum concentration (Cmin). The Cmin must remain above the MEC and well above the IC50 to ensure the drug's effectiveness.
Another related term is the trough concentration (Ctrough), the concentration of a drug in the blood immediately before the next dose is administered. ("Trough" is pronounced "troff.") The Ctrough may be higher than the Cmin, to account for the fact that it can take some time for a second dose of the medication to halt and reverse the dropping concentration associated with the first dose of the medication. This helps to ensure that the drug concentration remains above the MEC.
The area-under-the-curve (AUC) is the total amount of drug maintained in the blood. Also of importance is the drug's half-life -- the amount of time it takes the drug concentration to decrease by 50% in the blood or, even more importantly, inside cells (the intracellular half-life). Knowing this helps researchers determine the number of doses needed in a 24-hour period, such as once, twice, or three times a day.
Evaluating these PK characteristics not only allows researchers to establish an initial dose for a drug, but also allows them to make new dosing recommendations when necessary. The development of resistance to an HIV drug is a prime example.
As HIV accumulates mutations that cause resistance to a particular HIV drug, its IC50 increases, meaning that more drug is needed to decrease HIV replication by 50%. In turn, its MEC increases as well. Higher concentrations of the drug need to be maintained in the body to control the mutated virus. This might be achieved by increasing the dose of the drug, but this can increase the drug's Cmax, resulting in unacceptable side effects.
Another possibility might be to increase the number of times the drug is given each day, but this can be difficult for many people. A third solution, which has become very common, is to use Norvir to decrease the metabolism of other PIs, resulting in blood concentrations that are high enough to maintain suppression of the virus without the need for more frequent dosing.
Aside from being a fascinating science, PK research has provided healthcare providers and people living with HIV with important information to help them decide how best to use medications safely and effectively. While PK research continues to play a significant role in the development of new HIV medications, it is also playing a major role in ongoing research of older medications.
Finding ways to simplify treatment and increase drug concentrations to effectively treat drug-resistant HIV without unnecessary side effects are just two examples of how this research helps to shape the various treatment options available to people living with HIV. In this sense, not only is PK research helping to create new HIV medications, it is also helping create important alternatives using the options we already have.
Tim Horn is Executive Editor of The PRN Notebook and Senior Editor of AIDSmeds.com. He wishes to acknowledge the editorial support and guidance of John Gerber, M.D., of the University of Colorado Health Sciences Center and David Back, Ph.D., and Laura Dickenson, B.Sc., of the Liverpool HIV Pharmacology Group.