In this round of #AmyloidosisJC, we will be discussing a basic science paper that describes small molecules that stabilize light chain proteins in vitro. The following summary was written by Gareth Morgan (@wittyremarkhere), the first author. He will lead the discussion for this session of #amyloidosisJC.
Here is a link to the original paper:
This work was carried out at Scripps Research by the group of Dr. Jeff Kelly.
Dr. Kelly’s group invented tafamidis, a molecule that stabilizes transthyretin that was recently FDA approved (link: https://www.pfizer.com/news/press-release/press-release-detail/u_s_fda_approves_vyndaqel_and_vyndamax_for_use_in_patients_with_transthyretin_amyloid_cardiomyopathy_a_rare_and_fatal_disease) for treatment of ATTR amyloidosis.
Amyloidosis is caused by aggregation of normally-soluble proteins. In inherited diseases such as familial ATTR amyloidosis, aggregation is linked to destabilization of the precursor protein by mutation. In AL amyloidosis this connection is less well understood, because each patient has a unique amyloid-forming antibody light chain, secreted by monoclonal plasma cells. However, several lines of evidence show that unstable light chains are associated with amyloidosis:
1. Hurle et al 1994 https://www.ncbi.nlm.nih.gov/pubmed/8202506
2. Blancas-Mejia et al, 2014 https://www.ncbi.nlm.nih.gov/pubmed/24157440
3. Morgan and Kelly 2016 https://www.ncbi.nlm.nih.gov/pubmed/27569045
4. Brumshtein et al 2015 https://www.ncbi.nlm.nih.gov/pubmed/26576950
Since stabilization of precursor proteins has been shown to have clinical benefit for ATTR patients, the authors looked for stabilizers of light chains.
Key points from the paper:
1. The authors developed a method to measure light chain stability by high throughput screening. This was important because light chains do not have any known natural ligands that could be modified to make a drug. Instead, screening a large number of molecules was required.
2. From a starting set of 650,000 molecules, 16 molecules in four chemical classes could stabilize light chains when tested in several assays.
3. The paper focuses on one compound, which is a commercially available dye called “coumarin 1”. This molecule becomes fluorescent when it binds to light chains, which makes it useful as a tool for other experiments.
4. The crystal structure of coumarin 1 bound to light chains shows that the small molecule binds between the two variable domains in the dimer, at an interface that is made up of highly conserved residues (Figure D in the image seen at this link: F3.large.jpg). This site is likely to be present in most patient’s involved light chains. However, the molecules do not bind to the normal antibody heavy chain:light chain dimer interface.
5. The authors intend to develop molecules that bind more tightly and more specifically to light chains. These molecules could become drug candidates.
Clinical points for discussion:
1. These molecules are not drugs. There is a lot of work to be done before they can be tested in patients.
2. Stabilization may be most effective in when combined with anti plasma-cell therapies. One potentially promising use would be in maintenance for patients who have a hematological response to therapy but are at risk of relapse. Another would be for patients who are too sick to tolerate cytotoxic drugs.
3. Doxycycline, which has shown some efficacy for AL in Phase 2 trials, does not stabilize light chains in this assay – whatever it’s doing is probably different. https://www.ncbi.nlm.nih.gov/pubmed/28338670
4. Tafamidis is beneficial in ATTR patients. Would a light chain stabilizer have similar properties? We don’t know what the consequences of stabilizing light chains in patients will be. Many individuals with other plasma cell dycrasias (e.g., MGUS, smoldering myeloma, multiple myeloma) tolerate elevated levels of a monoclonal light chain without direct organ damage. However, light chains are cleared by the kidneys and several renal syndromes other than amyloidosis are associated with light chains. Altering light chain metabolism may cause problems in the kidney or elsewhere.
5. if stabilizers alter light chain clearance, they may interfere with free light chain measurements, currently considered key in the management and monitoring of AL amyloidosis.
6. It may be possible to measure the inherent stability of light chains in blood, thereby potentially identifying patients who might benefit from light chains stabilization by small molecules.