Q&A With Claude Wischik. Questions by Esther Landhuis.
Q: The ClinicalTrials.gov entry says placebo tablets will contain 4 mg LMTX as "colorant to maintain blinding." Does this truly maintain blinding? It seems that 100 mg drug would appear darker blue, while the 4 mg in the placebo tablet would come out as a lighter blue color in urine. Has this been tested? Are the actual drug and placebo pills, in fact, indistinguishable in color when they come out in urine/feces?
A: We have undertaken dose-ranging studies and measured the degree of urinary and fecal discoloration. Urinary discoloration is not dose-dependent (DiSanto and Wagner, 1972). We have been able to demonstrate sufficient overlap in terms of discoloration of excreta to permit the blind to be maintained. This point was discussed with and approved by the FDA.
Q: It is unusual for clinical trials to mix drug into the placebo pill. Did the company try food coloring (or some other form of colorant besides the drug itself) to satisfy blinding?
A: We did test a range of potential food colorants. The parent proved to be most reliable and appropriate.
Q: Will the trial use any biomarkers, such as CSF tau measurement, to verify target engagement?
A: There are still many unanswered questions about what the cerebrospinal fluid (CSF) tau measurement biomarker is showing, and how precisely this correlates to verification of target engagement. The assay claims to measure phosphorylated tau (p-tau), and doesn’t appear to measure the tau aggregation, which LMTX is targeting. P-tau is very proteolytically susceptible, and it is unclear how such an antigen could get to and survive in CSF. The only tau released from extracellular tangles is a short fragment from the core of the tau molecule (Mena et al., 1995). Furthermore, the CSF tau measurements only look at antibody immunoreactivity, and we do not know as yet exactly what protein species is being measured. Until it has been sequenced, we can’t even say for sure it is tau. Until these questions are answered, it could potentially be misleading to use such biomarkers to evaluate the efficacy of LMTX in clinical studies.
Increased levels of total tau and p-tau and decreased levels of Aβ immunoreactivity in CSF are indicative of AD, and in combination they can provide good sensitivity and specificity for diagnosis. Nonetheless, evidence that changes in the levels of these CSF biomarkers follow the clinical progression of the disease is lacking. The corollary to this is that decreases in CSF tau may not reflect cognitive improvement. Recent data from the EFNS Stockholm meeting supporting the engagement of bapineuzumab with its target based upon changes on CSF Aβ in the absence of clinical efficacy have been discussed on Alzforum (ARF related news story) and have been treated with mixed reactions. It is not clear why a drug targeting amyloid should result in a lowering of CSF tau immunoreactivity.
It is assumed that a drug that prevents tau aggregation and dissolves tau aggregates will lead to the release of tau into CSF. This is untested, and several other possibilities exist that would refute such a hypothesis. The source of tau may be extracellular, and its removal may only affect transneuronal spread within the brain, whereas a major objective for TAI would be to prevent intracellular aggregation of tau before there is loss of neuronal function on its way to becoming an extracellular tangle.
Antibodies to different phosphorylated and non-phosphorylated tau epitopes were used to identify three histologically distinct types of neurofibrillary tangles in Alzheimer's disease by Bondareff et al. (Bondareff et al., 1994). Intracellular tangles (Type 1) were identified by antibodies recognizing epitopes throughout the tau molecule, including the amino-terminus.
Compact extracellular tangles (Type 2) were characterized by the loss of amino-terminal and, hence, phosphorylation-dependent immunoreactivity, but retention of other tau epitopes. Dispersed extracellular tangles (Type 3) were characterized by the presence of epitopes associated only with the microtubule binding region and the C-terminus. The three stages of neurofibrillary degeneration can be understood as a sequential stripping of the proteolytically sensitive fuzzy outer coat of the paired helical filament, leaving behind the proteolytically stable core which comprises solely the repeat-domain core fragment that constitutes the core structure of the PHF (Wischik et al., 1988). Phosphorylated tau epitopes are located exclusively in the fuzzy outer coat of the PHF. Thus, when they become extracellular after neuronal death, the neurofibrillary tangles are non-phosphorylated (Bondareff et al., 1994), and so clearance of p-tau into CSF from this source is not possible. It is, therefore, difficult to understand in terms of an underlying mechanism how presence of p-tau in CSF is a marker of neurodegeneration.
The regulators have not accepted any validated biomarker for AD, but are obviously keen that such a biomarker or combination of biomarkers will eventually be validated. There are, however, many unanswered problems in relation to CSF biomarkers. Sample timing, collection, storage, and transport introduce variability. The nature of the analyte to be measured is still being investigated, with antibodies to different epitopes being used in assays and novel assays still under development.
Our PET and SPECT findings from the Phase 2 trial suggest that such molecular imaging may be a more direct biomarker of disease progression, and we are including imaging as an endpoint in one of our Phase 3 AD trials. Indeed, TauRx is developing novel imaging markers to permit direct detection of aggregated tau in the brain by PET imaging.
Q: Did the company do dose-ranging to determine which doses reach the brain and engage the target, and generally satisfy CNS PK/PD requirements? This has become standard for drug trials in the field.
A: We have undertaken extensive animal studies and Phase 1 studies in humans. We have determined plasma and brain levels in minipig, which is the most representative species for PK purposes and for measuring MT absorption and disposition. We have also undertaken radiolabeled MT distribution studies, including brain in minipig.
Finally, we have determined efficacy on tau pathology in two different tau transgenic mouse models that produce tau pathology. We have also measured the brain concentration at which effect on tau pathology is observed, and have related this back to plasma levels in humans, and plasma and brain levels in minipigs.
Q: The 12-month Phase 3 in mild to moderate AD and the 18-month Phase 3 in mild AD are slated to begin enrolling in October, according to the ClinicalTrials.gov postings. Correct?
A: We are on track for 2012, and will make an announcement regarding the AD trial program soon.
Q: The Phase 2 study in people already taking medications for mild to moderate AD began enrollment in August, according to the ClinicalTrials.gov postings. Correct?
A: Yes. This is a small safety study to determine the safety of co-medicating with LMTX and pre-existing stable acetylcholinesterase inhibitor and/or memantine therapy in mild to moderate Alzheimer’s. The study has regulatory approval and is due to start dosing shortly in the U.K. and Germany.
Q: You mentioned that the $170 million TauRx has raised thus far, plus the $120 million committed, will cover Phase 3. Do you mean all three trials in bvFTD and AD, or only the FTD one?
A: We have enough raised/committed to cover all three trials in their basic form as currently planned. We plan further fundraising.
Q: Most of the funding comes from private investors, not the more typical drug development investors, i.e., pharma or the biotech venture capital industry. Correct?
A: Most of the funding for our Phase 3 FTD and AD trials comes from private and institutional investors. We have a small discovery partnership in the field of development of novel tau imaging markers, which was with Bayer Healthcare and, following their divestment of this division, is now with Piramal Imaging, with whom we are currently in discussions. We have no immediate plans or needs to partner with a major pharma company. At the moment, we are able to carry the bulk of the risk of our own programs. We always remain open to discussion.
Q: Except for a 2009 Psychopharmacology paper, a mouse study on MTC/rivastigmine, and some conference abstracts, I found no publications on preclinical or clinical development of Rember or LMTX. Correct?
A: We will consider publishing some of the extensive body of work on preclinical and clinical development for the general scientific audience in due course. We published seven abstracts relating to preclinical and clinical development on Rember for ICAD in 2008. Reviews originally published as book chapters are available on the TauRx website. Our objective has been completion and presentation of full reports of the preclinical and clinical development program for the purpose of regulatory approval of the Phase 3 program.