**10. What needs to be kept in mind when measuring Lp(a)**

**8.2. CETP inhibitors**

146 Lipoproteins - From Bench to Bedside

are still lacking.

hyper-Lp(a).

considerably (see ref. [16]).

occasions.

**9. How and when should Lp(a) be measured**

indication, high Lp(a).

CETP stands for "cholesterol ester exchange/transfer protein." It catalyzes the exchange of CE and triglycerides between VLDL or LDL and HDL. In fact, many years ago, we published that Lp(a) also serves as a substrate for CETP [35]. On theoretical grounds, CETP inhibitors should be ideal for the treatment of stroke patients that exhibit significantly elevated Lp(a) and reduced HDL levels [36]. The development of drugs containing the CETP inhibitors Torcetra‐ pib and Dalcetrapib has been stopped because of unwanted side effects. Anacetrapib and Evacetrapib, on the other hand, are currently in phase II clinical trials. Concerning Anacetrapib, it was published in several reports that it reduces Lp(a) by up to 25%, yet details of this study

Further medications such as Eprotirome, a thyromimetikum, Lomitapide, an MTP inhibitor from Aegerion, and Mipomersen, an antisense oligonucleotide targeting apoB, all reportedly reduce Lp(a); however, it is rather uncertain that these drugs will ever be admitted for the

The most promising medication, at the time being, appears to be APO(a)rx, a specific antisense drug from ISIS®. In that respect, it is noteworthy that we published in 2001 that by RNA interference, a 100% inhibition of the expression of ap(a) may be accomplished in transgene apo(a) mice [39]. ISIS®, in fact, claims from a phase I study that in patients with Lp(a) of 10– 100 mg/dl, a reduction of up to 90% was reached (http://www.isispharm.com/Pipeline/ Therapeutic-Areas/Cardiovascular.htm#ISIS-APOARx). If ISIS ® succeeds in admitting their antisense drug APO(a)rx, we consider this strategy as the most specific and effective in treating

Actually, Lp(a) is not measured routinely in clinical laboratories because of the following reasons: (i) there exists no effective treatment regime to lower plasma Lp(a); (ii) the currently available Lp(a) assays are not standardized and results from different laboratories vary

**i.** As mentioned above, very effective and partially specific Lp(a) medications are in

**ii.** It is true that most commercial Lp(a) assays are not standardized, and the accuracy

and precision of these assays needs to be seriously revised. Since there is a strong

clinical trials and it is hoped that some of them might soon be on the market. Even if it should take several years before such drugs are admitted, knowledge of the plasma Lp(a) value gives additional important information to judge CHD risk. It has been reported previously that plasma Lp(a) levels stay pretty constant over months and years and may hardly be influenced by diet and living conditions. Systematic studies within single individuals, however, revealed quite large fluctuations. Patients with elevated or borderline Lp(a) values therefore should be assayed for Lp(a) at several

The first laboratory methods for measuring plasma Lp(a) were radial immune diffusion, rocket electrophoresis and later nephelometry. Today, high-throughput methods are mostly request‐ ed comprising ELISE, DELFIA nephelometry and turbidimetry. In all these methods, one must consider the fact that the molecular mass of Lp(a) and apo(a) varies quite strikingly within large limits, that Lp(a) contains apoB-100 in addition to apo(a), that Lp(a) exhibits great affinities to other proteins, e.g. β2-GPI, and in particular that apo(a) contains repetitive structures: the number of repetitive K-IV repeats varies from 2 to approximately 40 or more. This causes, in many assays, an overestimation of Lp(a) concentrations in patients with large apo(a) isoforms and an underestimation in patients with small apo(a) isoforms. Finally, one must consider the presence of small apo(a) fragments in plasma that are not bound to LDL. Yet the concentration of these fragments correlates positively with Lp(a) levels. In order to circumvent some problems in the quantitative analysis of Lp(a), ELISA and DELFIA methods have been suggested where the capture antibody binds relatively unspecific all apo(a) isoforms yet the detection antibody is monoclonal and recognizes only one epitope in apo(a). Other assays use anti-apoB as a detection antibody. This, however, is biased by the fact that in hyperlipidemic samples, one Lp(a) particle may bind additional apoB-containing lipoproteins leading to an overestimation of plasma Lp(a) levels.

As a consequence of these challenges, a group of experts in the field vent together that tackled these problems by propagating various reference standards and methods. We also participated in this survey using our in-house laboratory methods and antibodies [40]. A major problem that came out from this study was the different reference materials used in the assays. Even the use of the WHO/IFCC Reference Material as a common calibrator did not result in satisfactory harmonization of Lp(a) values [41].

We consider most of the considerations, based on theoretical grounds, of little importance in commercial routine clinical assays. There are three important questions that need to be solved: (1) What methods are apo(a)-isoform insensitive? (2) How can be units in mg/dl be transformed into nmol/L? (3) What are the cutoff levels to be adopted for risk stratification?
