Is There a Threshold for Medication Adherence? Lessons Learnt From Electronic Monitoring of Drug Adherence

Medication adherence is a well-recognized problem in the management of patients with chronic diseases needing a long-term pharmacotherapy. While fighting against non-adherence, an important question frequently arises, i.e., how much adherence is enough to obtain the full treatment benefits? Most studies having attempted to answer this question have used large pharmacy records and health care databases to quantify the percentage of days covered by the prescribed treatment and hence deduce a threshold below which there appears to be fewer benefits from therapy. In the present paper, the use of data obtained using electronic monitoring of adherence is discussed as another means to assess adherence thresholds with a particular emphasis on hypertension. The data show that even with the use of electronic monitoring of adherence, which provides a comprehensive dosing history, it is extremely difficult to define an adherence threshold in hypertension. This is due to many factors that need to be taken into account, including not only the pattern of patients’ adherence and their clinical and environmental characteristics, but also the pharmacological characteristics of the prescribed drugs, the severity of the disease and many others. To determine adherence cut-offs more precisely, specific protocols should be designed to answer the question in various clinical conditions. These protocols should be conducted in well-defined patients’ groups, they should use the most reliable methods to measure adherence providing if possible a detailed dosing history perhaps combined with drugs levels in blood or urine. These studies should also choose the best methods to measure clinical endpoints, such as ambulatory blood pressure monitoring or home blood pressure in the case of hypertension. One important aspect is that datasets should be solid and large enough to be able to analyze adherence data as a continuous variable using newly developed mathematical models including new metrics catching the complexity of adherence. The rapid development of new technologies like devices, connectivity, and analytics, will probably provide new solutions to improve our ability to define valid and clinically useful adherence thresholds in various therapeutic areas.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

An automated electrophysiology serum shift assay for K(V) channels

The presence of serum in biological samples often negatively impacts the quality of in vitro assays. However, assays tolerant of serum are useful for assessing the in vivo availability of a small molecule for its target. Electrophysiology assays of ion channels are notoriously sensitive to serum because of their reliance on the interaction of the plasma membrane with a recording electrode. Here we investigate the tolerance of an automated electrophysiology assay for a voltage-gated potassium (K(V)) channel to serum and purified plasma proteins. The delayed rectifier channel, K(V)2.1, stably expressed in Chinese hamster ovary cells produces large, stable currents on the IonWorks Quattro platform (MDS Analytical Technologies, Sunnyvale, CA), making it an ideal test case. K(V)2.1 currents recorded on this platform are highly resistant to serum, allowing recordings in as high as 33% serum. Using a set of compounds related to the K(V) channel blocker, 4-phenyl-4-[3-(2-methoxyphenyl)-3-oxo-2-azaprop-1-yl]cyclohexanone, we show that shifts in compound potency with whole serum or isolated serum proteins can be reliably measured with this assay. Importantly, this assay is also relatively insensitive to plasma, allowing the creation of a bioassay for inhibitors of K(V)2.1 channel activity. Here we show that such a bioassay can quantify the levels of the gating modifier, guangxitoxin-1E, in plasma samples from mice dosed with the peptide. This study demonstrates the utility of using an automated electrophysiology platform for measuring serum shifts and for bioassays of ion channel modulators.