http://www.chemsilico.com/CS_prPB/PBintro.html
ChemSilico, CSPB
Protein Binding, Introduction.

Protein binding values (% fraction bound) are normally given as the percentage of the total plasma concentration of a drug that is bound to all plasma proteins.

In most cases, binding to plasma proteins is reversible ....

The protein samples used in binding studies are taken from healthy individuals because there are several disease states that can affect the protein binding of drugs. Examples include hepatic diseases that alter the concentration of albumin in the plasma and uremia that can cause changes in the binding affinity of some drugs.

Depending upon whether the drug is a weak or strong acid or base, or is neutral, it can bind to a single blood protein, to multiple proteins (e.g. serum albumin, acid-glycoprotein (AGP) or lipoproteins). The most significant protein involved in the binding of drugs is albumin, which comprises more than half of all blood proteins. Albumin can interact with acidic or basic drugs in the plasma by van der Waals dispersion forces, hydrophobic bonding, hydrogen bonding, and ionic interactions (2). AGP interacts with mainly with basic entities whereas lipoproteins bind both basic and neutral drugs.

The extent to which a drug is bound to plasma proteins can affect the distribution of the drug in several ways. The drug/protein complex does not permeate phospholipid bilayers, including capillary membranes, glomerular membranes in the nephrons, and the blood brain barrier (2). Bound drugs are also less available to the enzymes involved in first-pass metabolism. After much of the free drug has been cleared by the metabolic and excretory processes, the reversible drug/protein complex serves as a depot to replenish the concentration in vivo.

... drugs with high protein binding activity values tend to have a greater half-life compared to those with lower values. The prolonged activity resulting from these factors may be desirable, or may promote the emergence of undesirable side effects.

2

http://sres.anu.edu.au/associated/fpt/nwfp/quinine/Quinine.html
Australian National University,
Quinine.

Quinine has many uses and applications.
Analgesic, anesthetic, antiarrhythmic, antibacterial, antimalarial, antimicrobial, antiparasitic, antipyretic, antiseptic, antispasmodic, antiviral, astringent, bactericide, cytotoxic, febrifuge, fungicide, insecticide, nervine, stomachic, tonic.

Quinine (and other drugs such as chloroquine) are effective in combating malaria because they are able to bind strongly to blood proteins, and form complexes which are toxic to the malarial parasite.

Malaria is caused by the Plasmodium parasite.
There are different strains of malarial parasites, and these vary in their severity of attack. One of the most deadly strains is Plasmodium falciparum. The Plasmodium parasite has a highly specialised and intricate life cycle, which involves both sexual and asexual stages, and so requires different hosts for completion. The asexual stage occurs in humans, whilst the asexual stage occurs in mosquitoes (Campbell, 1993).

3

http://dreampharm.com/Item/103.htm
DreamPharm,
Coenzyme Q10.

Coenzyme Q10 (also known as ubiquinone) ...

... health benefits of taking CoQ10, which include the improvements in the heart function and circulation for those with congestive heart failure, weakened heart muscle, high blood pressure, heart rhythm disorders, and chest pain (angina). Studies suggest CoQ10 may also help to protect against blood clots, heart attacks, chronic fatigue, and other problems linked to damages from free radicals such as aging and cancer, and maintain healthy teeth and gums. Since CoQ10 is a strong metabolic catalyst and antioxidant, it may also protect the nerve cells (neurons) and help slow Alzheimer's or Parkinson's disease.

Anti-oxidants protect our body from damages caused by toxic free-radical chemicals generated in our body or of environmental origin. Anti-oxidants thus slow wear and tear of our body (aging) and reduce chances of illnesses. Anti-oxidants are strongly linked to longevity. Deficiency of anti-oxidants may not necessarily cause illness in short terms. However, in the long run, it will accelerate aging and cause many illnesses.

4

http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/
journal/v408/n6811/abs/408479a0_fs.html
Binding of disease-associated prion protein to plasminogen
by MICHAEL B. FISCHER, CHRISTIANE ROECKL, PETRA PARIZEK, HANS PETER SCHWARZ & ADRIANO AGUZZI

Transmissible spongiform encephalopathies are associated with accumulation of PrPSc, a conformer of a cellular protein called PrP C. PrPSc is thought to replicate by imparting its conformation onto PrPC (ref. 1), yet conformational discrimination between PrPC and PrPSc has remained elusive. Because deposition of PrPSc alone is not enough to cause neuropathology, PrPSc probably damages the brain by interacting with other cellular constituents. ...

We identify plasminogen, a pro-protease implicated in neuronal excitotoxicity, as a PrPSc-binding protein. Binding is abolished if the conformation of PrPSc is disrupted by 6M urea or guanidine. The isolated lysine binding site 1 of plasminogen (kringles I–III) retains this binding activity, and binding can be competed for with lysine. Therefore, plasminogen represents the first endogenous factor discriminating between normal and pathological prion protein.

5

http://www.usm.maine.edu/psy/markowski/Drug_Disposition.html
University of Southern Maine,
Drug Disposition.

III. Distribution

A. Circulation: entire blood volume rotation is rapid.
1. Drug distribution to other organs may produce side effects.
2. Certain proteins in blood bind to drugs, inhibiting site interactions.

B. Crossing permeability barriers:

1. Cell membrane: phospholipid bilayers
   

   a. A drug can diffuse passively -
   b. or be transported.

2. Capillary: H2O-filled pores are exit sites for drugs.

3. BBB: in the brain, capillary pores are “sealed” by glia.
   a. Only highly lipid-soluble drugs penetrate BBB.

4. Placenta: regulates nutrient, waste, drug exchange between fetus and mother.

IV. Elimination

A. Kidney Function:
renal excretion is the predominant elimination mechanism.

B. Liver Function:
biotransformation is often a prerequisite of elimination by kidneys.

1. Numerous factors affect elimination: age, sex, genetic predispositions, exposure to environmental compounds, disease

2. 2. A relatively small group of liver enzymes can metabolize many chemicals.

C. Lactation

6

http://www.ptcommunity.com/ptjournal/fulltext/28/7/PTJ2807468.pdf
Burke A. Cunha, MD,
Principles of Antibiotic Formulary Selection
for P&T Committees.
Part 2: Pharmacokinetics and Pharmacodynamics
Vol. 28 No. 7 • July 2003 • P&T ® 469

After an antimicrobial agent is absorbed into the blood- stream by any route, it is reversibly bound to serum albumin, the predominant protein in the blood. Protein binding is expressed as a percent, and it represents the percentage of the antibiotic that is reversibly bound to serum albumin. Anti- biotics also bind to tissue sites, but protein binding varies according to the type of tissue; this facet has not been well studied.

The serum compartment, which contains the antibiotic reversibly bound to serum albumin, is transported via the circulation to all body sites. Moving along a concentration gradient, the antibiotic dissociates itself reversibly from albumin and penetrates a series of membrane barriers to reach the site of infection. After traversing a variable number of membranes, depending on the tissue being penetrated, the antibiotic becomes reversibly bound to tissue proteins.

It is the free or unbound portion of an antibiotic in the serum or at the tissue level that is effective in microbial eradication. As the concentration in the tissue decreases, more of the antibiotic, if it is available from the serum compartment, moves along a concentration gradient into the tissues. When the tissue concentration falls, the concentration gradient is reversed and the antibiotic can move back into the serum compartment until it is eliminated if it is not metabolised at the tissue level.

The dose and dosing interval recommendations are based largely on peak serum concentrations and the beta elimination half-life to determine an appropriate dosing interval. Putting these three factors together, appropriate dosing provides for the interplay of these factors, which results in a concentration for a sufficient duration at the intended target site of infection.

Absorption
Some antibiotics are given via the intravenous (IV) or the intramuscular (IM) route because they are not well absorbed via the oral (PO) route. IV antibiotics achieve peak serum concentrations rapidly; IM antibiotics achieve somewhat lower serum concentrations but have more prolonged serum levels. The absorption of orally administered antibiotics is variable.

The relative absorption is termed bioavailability and is expressed as the percentage absorbed. High-bioavailability antibiotics (greater than 90%) are ideal for IV-to-PO switch programs, because serum/tissue levels are comparable. Antibiotics with low bioavailability (below 50%) are incompletely absorbed and are often associated with gastrointestinal side effects.

Route of Elimination
Most antibiotics are eliminated by hepatic or renal mechanisms. Some antibiotics are metabolized, although their precise mode of excretion is not entirely clear. Excessive amounts of drug in the serum compartment (or the amount of drug remaining after reaching tissue sites and returning to the serum compartment) are eliminated.

Most antibiotics are eliminated via the kidney and are excreted into the urine as active or inactive drug, plus or minus active or inactive metabolites. Most antibiotics eliminated via hepatic mechanisms are excreted into the bile and into the feces as active or inactive drug, plus or minus active or inactive metabolites.

The mode of elimination is also important in the treatment of urinary tract infections. Most antibiotics that are renally metabolized or inactivated are excreted into the urine at high concentrations. Antibiotics that are eliminated through the kidney, for the most part, are concentrated to supraserum levels in the bladder urine. This is therapeutically useful because some organisms that may appear to be resistant to antimicrobials at the usual serum concentrations may, in fact, be susceptible in the bladder to urinary concentrations of renally eliminated antibiotics.

The converse is also true: antibiotics that are eliminated hepatically (e.g., moxifloxacin HCl [Avelox®, Bayer]) usually do not achieve adequate urinary concentrations. Therefore, if a quinolone is selected to treat cystitis, then ciprofloxacin (Cipro®, Bayer), ofloxacin (Floxin®, OrthoMcNeil, levofloxacin (Levaquin TM , Ortho-McNeil), or gatifloxacin (Tequin®, Bristol-Myers Squibb) should be used instead of moxifloxacin.

Tissue Penetration
Infectious diseases occur either in the bloodstream, as with bacteremia, or, as in most cases, in a particular organ site (e.g., the lungs) or in a target tissue (e.g., middle-ear fluid in patients with otitis media). When planning for effective antimicrobial therapy after the appropriate spectrum of coverage has been selected, we would next consider the agent’s pharmacokinetic properties.

Antimicrobial agents with a proper spectrum are ineffective if they cannot reach the site of infection. Hence the phrase “tissue is the issue” summarizes the importance of pharmacokinetics in antibiotic selection. With the appropriate dosing interval used to ensure concentration at the target tissue, the chosen antibiotic must be administered by a route and dose that are in excess of the minimal inhibitory concentration (MIC)

Microbiologic Activity versus Antibiotic Concentration Pharmacokinetic and activity relationships are also important in other clinical situations. For example, aminoglycosides have suboptimal antimicrobial activity in areas of local hypoxemia, increased cellular debris, and local acidosis. This is the case within the lungs with pneumonia and in other situations such as osteomyelitis.

With respect to pneumonia, gentamicin (Elkins-Sinn) achieves high concentration in pulmonary tissue; however, it is probably not the best choice as monotherapy in pneumonia even when the infection is caused by susceptible organisms, because the activity of gentamicin and other aminoglycosides is greatly diminished in the presence of the factors explained earlier.

Vancomycin (Vancocin®, Eli Lilly) and the macrolides are large molecules that do not penetrate well into synovial fluid in patients with septic arthritis, simply because of their considerable molecular size. Other agents with an appropriate spectrum should be used in place of these agents if the infection involves the synovial fluid.

Protein Binding
High serum protein binding (over 90%) was initially thought to be a disadvantage in an antibiotic. Conceptually, high protein binding means that the antibiotic is bound to serum proteins and is not available in the unbound (free) state to attack the infecting microorganisms. Intuitively, it would seem that antibiotics with a low protein binding (10%) have an advantage of being nearly all free and available for microbial killing. It has also been noted for many years that an increase in protein binding increases the MIC against certain organisms (e.g., Staphylococcus aureus) with certain antimicrobials. All of this suggests that low protein binding is preferable.

Actually, however, the answer to the question depends on the achievable serum concentrations. For example, when cefazolin (e.g., Ancef®, GlaxoSmithKline), which is 86% protein-bound, is compared with cephradine (Velosef®, Apothecon), which is 10% protein-bound, cefazolin on a gram-for-gram basis achieves higher blood serum concentrations than does cephradine. The serum concentrations after administration of 1 g of cefazolin (approximately 200 mg/ml) are sufficient to saturate the albumin-binding sites in the serum compartment and to still have an excess of free drug available in the serum for penetration into tissue sites. If we measure the amount of free antibiotic of cephradine versus cefazolin in tissue using a 2-g (IV) dose for each as a comparison, the free levels with cefazolin (high protein binding) are still higher than with cephradine.

....

The high-protein binding to serum albumin acts as an “antibiotic reservoir” and reversibly releases antibiotics from the binding sites as serum and tissue concentrations decrease. This represents the equivalent of a depot formulation of an antibiotic that slowly releases antibiotic back into circulation after serum concentrations have fallen from serum-binding and tissue-binding sites.

....

Volume of Distribution
The volume of distribution (Vd) represents the “apparent” volume into which an antibiotic is distributed. This value is derived by the amount of antibiotic in the body divided by the serum concentration (L/kg). The Vd is related to total body water distribution (VdH2O = 0.7 L/kg). Water-soluble (hydrophilic) antibiotics are limited to extracellular fluid and have a Vd of 0.7 L/kg or less. Highly soluble (hydrophobic) antibiotics penetrate most body tissues well because of their large Vd.

Most tissues are rich in lipids and are well penetrated by drugs with a high Vd. The Vd can be affected by organ profusion, lipid solubility, protein binding, and membrane diffusion or permeability. It can be increased in certain patient subsets with hydrophilic drugs, such as patients receiving dialysis, those with cirrhosis, those undergoing mechanical ventilation, patients with burns, or patients with heart failure.

Decreases in the Vd for hydrophilic drugs can occur with pancreatitis, early loss of gastrointestinal fluid, trauma, or hemorrhage. Increases in the Vd may require increased daily antibiotic dosing to maintain drug effectiveness, and decreases in the Vd resulting from various pathological states may require a decrease in drug dosing.

Concentration-Dependent Susceptibility
Susceptibility is concentration-dependent; the usually recommended dose is not only optimal for achieving therapeutic serum and tissue concentrations but is also necessary to achieve the full therapeutic effect. The killing of organisms can be represented as a sigmoid curve of concentration versus the percentage of susceptible organisms.

As an example, the usual dose of cefoxitin is 2 g (IV) every six hours. Some prescribers have tried to decrease the cost of antimicrobial therapy by decreasing the dose. Unfortunately, this measure also decreases the activity of the drug against the organism in its usual spectrum. Thus, given the 2-g (IV) dose of cefoxitin, approximately 85% of Bacteroides fragilis organisms would be inhibited. If the dose were cut to 1 g, however, only 15% of B. fragilis isolates would be inhibited. This is an inadequate amount if cefoxitin (Mefoxin®, Merck) is being used to treat an intra-abdominal or pelvic infection.

... Some patients and infections in certain body sites, however, may warrant dosing modifications.

For example, penetration into the central nervous system requires drugs with different physicochemical characteristics or dosing modifications to achieve therapeutic concentrations in the cerebrospinal fluid. Drugs that can be used in the usual dose to treat central nervous system infections include chloramphenicol, because of its high lipid solubility, and doxycycline (Vibramycin® Calcium, Pfizer), minocycline (Dynacin®, Medicis), linezolid (Zyvox TM, Pharmacia & Upjohn), and TMP–SMX (trimethoprim–sulfamethoxazole [Bactrim®, Roche; Septra®, Monarch]).

Another example is ceftriaxone sodium (Rocephin®, Roche), an antibiotic with high serum levels that, even when given in the usual dose, penetrates the cerebrospinal fluid in adequate concentration. Other drugs require a higher than usual dose to achieve adequate concentration in the cerebrospinal fluid, ...

Most other antibiotics do not penetrate the cerebrospinal fluid regardless of the dose given (e.g., cefazolin).

PHARMACODYNAMICS
Pharmacodynamics (PD) takes off where pharmacokinetics leaves off. Whereas pharmacokinetics concerns the disposition, metabolism, and elimination of antibiotics in the body, pharmacodynamics involves the effects of antibiotics when drug concentrations are subtherapeutic or nonexistent.

The after-effect of an antibiotic after it is no longer present in adequate serum or tissue concentrations has been called the post-antibiotic effect. This effect was first noted decades ago by Eagle. He described the inhibitory effects of penicillin long after it was no longer pharmacologically available to account for an effect on the susceptible bacteria. The Eagle effect (now renamed the post-antibiotic effect) differs for gram-positive and gram-negative organisms and varies with the antibiotic being used. Both pharmacokinetic and pharmacodynamic properties should be considered for optimal therapeutic effect.

Concentration-Dependent and Time-Dependent Killing
Over the years, it has been established that antibiotics exert their optimal effect on susceptible microorganisms either as a function of concentration over the MIC or as a function of time over the MIC. Antibiotics that kill optimally at high concentrations have what are called concentration-dependent killing kinetics (i.e., the higher the concentration, the more effective the microbial eradication).

Other antibiotics kill by time-dependent (non–concentration-dependent) killing kinetics. With these antibiotics, as long as the concentration is maintained slightly above the MIC over time, killing is maximal. With time-dependent killing antibiotics, there is no advantage to high serum concentrations, as killing is not increased.

For concentration-dependent antibiotics, the pharmacokinetic parameter that best describes this relationship is a ratio of the peak serum/tissue concentration over the MIC of the organism in question. This has been termed a kill ratio or inhibitory index; the higher the ratio, the more optimal killing with an antibiotic that utilizes concentration-dependent killing kinetics.

...

7

http://science-education.nih.gov/newsnapshots/TOC_Xeno/
index/Science_Activity_2/science_activity_2.html
Science-Education, National Inst. of Health,
Snapshots of Science & Medicine
Xenotransplantation.

Antibodies in transplant recipient's blood bind to antigens on the cells lining the transplanted organ’s capillaries. Complement proteins attach to the bound antibodies and activate. Complement activation generates membrane-attack complexes (protein complexes that poke holes in cell membranes). Membrane-attack complexes lyse (burst open) cells lining the capillaries of blood vessels in the xenograft. Destruction of blood vessels cuts off transport of nutrients and oxygen to the xenograft. Organ dies due lack of nutrients.

Introduction
The human immune system is our own “personal bodyguard.” Like any good defender, it must recognize “the enemy” — that wide range of viruses, bacteria, fungi, protozoa and other would-be pathogens that we encounter every day. At the same time, the immune system must be able to distinguish friend from foe. (A guard that instantly attacks anything that moves is probably not someone you’d want in your home.)

Our immune system is genetically programmed to recognize certain proteins on our cells (self) from the thousands of invading pathogens (non-self) trying to gain a foothold in our body. But how is the immune system able to differentiate between friend and foe?

Recognition of the Body’s Own Cells
Cells posses unique antigenic proteins on their membranes that are like fingerprints; no two people (except identical twins) have the same protein structures in their membranes. Our immune cells see these proteins as normal or “self.” But if our cells are transplanted into another person, they cause an immune reaction. T and B cells mount an all-out attack in an attempt to rid the body of these foreign proteins.

The Recognition Problem in Xenotransplants
In the case of xenotransplants (transplanting organs across species), the genetic separation between donor and recipient is even greater. Membrane-bound proteins are less similar, and the rejection of the organ is stronger, faster, and more complex. In particular, xenotransplants trigger a response called hyperacute rejection, which can destroy a transplanted organ within just a few hours.

Hyperacute rejection results from activation of a part of the immune system called the complement system, an array of proteins that circulates in the blood. When activated, the complement proteins bind together to form “membrane-attack complexes” that can poke large holes in cell membranes. The problem is that all mammals have a set of species-specific antigens on the surfaces of cells lining blood vessels. When antibodies circulating in human blood see these antigens from another species, they quickly bind onto them and activate the complement system. The membrane-attack complexes generated then destroy the blood vessels supplying the organ with nutrients. Once hyperacute rejection gets going, the organ usually doesn’t survive for more than a few hours.

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