Monoclonal antibodies & PCSK9 pathway: Therapeutic Implications for CVD
The safety of therapeutic monoclonal antibodies: Implications for cardiovascular disease and targeting the PCSK9 pathway.
Catapano AL, Papadopoulos N.
Atherosclerosis. 2013 Feb 8. doi: 10.1016/j.atherosclerosis.2013.01.044.
Although good results in lowering the incidence of CV events can be obtained by lowering LDL-C, part of the patient population fails to reduce their LDL-C to recommended levels, or does not tolerate the commonly used statins. Thus, new treatment strategies are required. One of the approaches currently being developed is the use of monoclonal antibodies (mAbs).
mAbs have become established therapies for a number of disorders, including cancer and immunological diseases. Much effort has already been dedicated to studying mechanisms of distribution, potency, specificity, immunogenicity and dosing frequency.
To date, the only mAb licensed for the treatment of CV-related disorder is abciximab, which is used after PCI and for unstable angina. A newly developed class of mAbs targets PCSK9 (see below), which has the potential to manage CV risk in patients in whom existing therapies fail to lower LDL-C.
We here summarise the main points outlined in this review.
mAbs can directly interact with a target and change its biological function or they can interact with cell surface receptors, which then induce immune-mediated downstream effects, such as cytotoxicity. Alternatively, when a mAb is bound to its targets, the complement system can be activated, with subsequent induction of cytotoxicity.
How do mAbs work?
First generation therapeutic mAbs were made in mice. Patients treated with murine immunoglobulins often developed human antimouse antibodies (HAMAs), resulting in rapid mAb clearance. A solution for this problem was the fusion of a mouse antigen-binding domain to a human constant domain. These chimeric and humanised mAb caused less HAMA formation, but also decreased binding affinity. More recently, fully human mAbs have been produced, which show less immunogenicity than murine and chimeric mAbs. Already nine fully human mAbs have been approved for therapeutic use in Europe and the United States.
How are they made?
The direct interaction with the target molecule and/or processes may result in adverse effects (AE). AEs could be due to the intended pharmacological action of the mAb, or mAbs may cause toxicity by interacting with the target antigen outside the target tissue. Furthermore, off-target, non-specific toxicity can be observed, such as hypersensitivity reactions thought to be related to immunogenicity of mAbs. This risk has greatly been reduced, but not entirely eliminated, by the development of fully-human mAbs.
Primary safety concerns
Overall, the risk of serious adverse drug reactions with mAbs is generally low. mAbs are generally well-tolerated and safety profiles seem relatively stable over time.
The first mAbs have been developed for treatment of CVD or its risk factors:
mAbs for the treatment of CVD
- Abciximab: inhibits platelet cross-linking and aggregation by targeting glycoprotein IIb/IIIa, thus reducing blood coagulability, to be used in patients undergoing PCI. It is a chimeric mAb, which likely explains its off-target, non-specific AEs. Specific AEs are also seen, such as increased risk of bleeding and thrombocytopaenia.
- PCSK9 mAbs: target proprotein convertase subtilisin/kexin type 9 (PCSK9). This serine protease is responsible for post-transcriptional regulation of LDL-receptor (LDLR) levels. PCSK9 normally binds to the LDLR and stimulates its internalisation and degradation. Reduced availability of LDLR leads to higher plasma LDL-C.
- Thus, blocking the interaction between PCSK9 and LDLR by mAbs targeted against PCSK9 will likely reduce CV risk in people with hypercholesterolaemie. It is anticipated that PCSK9 mAbs may also increase the lipid-lowering efficacy of statins.
Genetic variation in the PCSK9 gene can influence affinity to LDLR and thereby plasma cholesterol levels, with concomitantly altered coronary heart disease risk. Both gain-of-function and loss-of-function PCSK9 mutations have been described, leading to increased and reduced CV risk respectively.
Efficacy and safety of PCSK9 mAbs
Pre-clinical studies with several PCSK9 mAbs developed by several pharmaceutical companies showed a promising reduction of serum cholesterol in several species (rodents, primates).
Phase I trials in healthy subjects and hypercholesterolaemie showed that PCSK9 mAbs are generally well-tolerated and significantly reduce LDL-C (study details are discussed in the review).
The first phase II clinical studies have recently been completed, studying different dosing regimens as well as combination therapies with statins. Details are discussed in the review.