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Plasma protein binding: From discovery to development

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Abstract

The importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites has been the topic of significant discussion and debate amongst drug development groups over the past few decades. Free drug theory, which states that in absence of energy-dependent processes, after steady state equilibrium has been attained, free drug concentration in plasma is equal to free drug concentration at the pharmacologic target receptor(s) in tissues, has been used to explain pharmacokinetics/pharmacodynamics relationships in a large number of cases. Any sudden increase in free concentration of a drug could potentially cause toxicity and may need dose adjustment. Free drug concentration is also helpful to estimate the effective concentration of drugs that potentially can precipitate metabolism (or transporter)-related drug–drug interactions. Disease models are extensively validated in animals to progress a compound into development. Unbound drug concentration, and therefore PPB information across species is very informative in establishing safety margins and guiding selection of First in Human (FIH) dose and human efficacious dose. The scope of this review is to give an overview of reported role of PPB in several therapeutic areas, highlight cases where PPB changes are clinically relevant, and provide drug metabolism and pharmacokinetics recommendations in discovery and development settings.

Section snippets

INTRODUCTION TO FREE DRUG THEORY—WHY IS PLASMA PROTEIN BINDING IMPORTANT?

High concentration of proteins in plasma and the propensity of numerous drugs to bind to them have led drug development groups to recognize the importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites. Free drug theory (FDT) is being increasingly used to explain pharmacokinetics/pharmacodynamics (PK/PD) relationships1—FDT states that in absence of energy-dependent processes (uptake and efflux transporters, pH gradient), after

MAJOR DRUG BINDING PROTEINS IN PLASMA

Major drug-binding components in plasma are albumin (human serum albumin—HSA) (600 μM), α-acid glycoprotein (AAG) (12–30 μM), lipoproteins (γ-globulin), and erythrocytes of which drug binding to albumin and AAG has been well studied and extensively published over the past several decades. Although HSA has eight binding sites, capable of binding to endogenous compounds as well as xenobiotics with varying affinities, two major sites of HSA are primarily involved in binding ligands and show a bias

SIMPLE, THEORETICAL CONSIDERATIONS OF PPB

Plasma protein binding is believed to have a significant influence in the rate of drug diffusion between plasma and tissues (influx and efflux)9 and therefore influence clearance (Cl) and volume of distribution (Vdss) of drugs. Effect of PPB on Cl is dependent on major route of Cl of the drug and in case of hepatically cleared drugs, on the liver extraction ratio. For renally extracted drugs, Cl due to glomerular filtration and active secretion are dependent on fu as only unbound drug can be

Endocrinology/Metabolism

Plasma protein binding optimization proved to be very useful in the development of depeptidyl peptidase DPP-IV inhibitors, for the treatment of type 2 diabetes mellitus. First-generation lead compound, although very potent, needed much higher than expected plasma concentration (based on its in vitro potency) for efficacy in preclinical model.11 To understand the disconnect between potency and efficacy, when the in vitro assay was performed with mouse and human serum, compound A (Fig. 4) showed

Nonsteroidal anti-inflammatory drugs

Aspirin, naproxen, piroxicam, ibuprofen, phenylbutazone, indomethacin, flurbiprofen, ketoprofen, methyl salicylate, diflunisal (Fig. 16) are some of the well-known NSAIDS used in the clinic for the treatment of inflammation-related diseases, for example, rheumatoid arthritis and chronic pain. All NSAIDS, with the exception of aspirin, are extremely highly plasma protein bound, which impacts not only their PK properties (Vdss, Cl, and t1/2), but also the duration and magnitude of their efficacy.

Species-Dependent PPB

Although not very common, sometimes, a significant difference in PPB between species is encountered and this can sometimes have major implications in predicting/determining safety and tolerability.ce:list id="l0020">

  • This is well exemplified in the case of anticancer drug UCN-01 (Fig. 33). UCN-01 demonstrated much unexpected PK (low Vdss, low Cl) in humans that were not predicted from preclinical data (high Vdss and high Cl). This significant difference in PK between humans and preclinical

CLINICALLY RELEVANT PPB–DDI

Clinical relevance of PPB displacement, resulting in a clinically relevant DDI has been an area of much discussion and debate.3,10,61,110., 111., 112., 113., 114., 115., 116., 117. There are several examples in literature where PPB displacement was originally proposed to cause a clinical risk, especially those with warfarin, tolbutamide, and phenytoin (all with NTI), but recent evaluation of these DDIs revealed that inhibition of metabolism of the victim drugs by the corresponding perpetrator

METHODOLOGY

Several different PPB methods have been reported to in literature, a brief summary of which is provided inTable 5.

The “gold standard” methods for PPB are equilibrium dialysis, ultrafiltration and untracentrifugation, with gel filtration method being very popular as well. Readers are encouraged to refer to some excellent reviews.38,127 Some of the major methods are briefly discussed below:

EXCEPTIONS TO FDT

Very simply, this can be described as when unbound drug in a compartment does not correlate to target receptor occupancy within that compartment, leading to a disconnect between unbound drug level and pharmacological activity. This is possible when certain key underlying assumptions of FDT are not met10,61,117:

  • (a)

    drug has attained steady state between plasma (plasma protein binding) and target tissue (receptor and site of action), but there are certain special cases when SS equilibrium is not

SUMMARY—DISCOVERY RESEARCH TO PRECLINICAL DEVELOPMENT STAGE

There have been extensive publications highlighting PPB optimization in drug discovery. It is widely believed that compounds that show less than 85% bindings are of little concen.95 However, “one glove fits all” theory does not work during lead optimization stage, that is, there is no “optimum/desired/cutoff” value of PPB that will ensure success of a chemical series to be successful. There are some misconceptions surrounding PPB, which are briefly discussed below:

  • Lowering PPB and optimizing

SUMMARY—CLINICAL DEVELOPMENT STAGE

It is very useful to consider several aspects of PPB in clinical situation, as learnt from the reports sited in literature and some of the important ones are discussed below:

  • DDI due to PPB displacement. To precipitate a DDI due to PPB displacement between two drugs, both drugs have to be (a) very highly bound and exceed concentration of the plasma protein that is (are) involved in binding of the drugs. For example, HSA concentration is approximately 600 μM, so even if two highly bound drugs

RECOMMENDATIONS (DISCOVERY RESEARCH TO PRECLINICAL DEVELOPMENT STAGE)

In early stages of drug discovery (lead identification/lead optimization stages), significant resources to perform detailed PPB evaluation is not recommended and a simple approach as outlined in Figure 52 is advised to be adopted. Final decision regarding PPB optimization strategy and assay type should be decided by drug discovery projects after thorough evaluation of project requirements, as it must be emphasized that optimum/ideal PPB values can be very different from one chemical series to

RECOMMENDATIONS (CLINICAL DEVELOPMENT STAGE)

Role of PPB on efficacy models needs to be rigorously evaluated and established throughout the preclinical and clinical development stages. Although a general scheme is outlined in Figure 53, it is up to the PK scientists to evaluate the PPB-efficacy relationships to design the best studies addressing this issue on a case-by-case basis. It is crucial to understand the differences (if any) of PPB between healthy (population in which therapeutic concentration range was initially investigated in)

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