Mixed lineage kinase inhibitor CEP-1347 fails to delay disability in early Parkinson disease

We read the Parkinson Study Group's article with interest. [1] The disappointing result in PRECEPT that used mixed-lineage kinase (MLK) inhibitor CEP-1347 for treatment of Parkinson Disease (PD) may be because: 1) the hypothesis that MLK inhibitors retard disease progression is incorrect; 2) the inhibitor does not reach therapeutic levels in the central nervous system; or 3) MLK inhibitors require an additional signal to maintain dopaminergic neuronal survival. Based on our research into the molecular mechanism of MLK inhibitors, we propose the third possibility as why MLK inhibitors did not maintain neuronal survival.

Small-molecule MLK inhibitors such as CEP-1347 and CEP-11004 inhibit activation of the c-Jun NH2-terminal kinase (JNK) and neuronal cell death pathway in many cell culture and animal models. However, in three well-defined cell culture models of trophic factor-withdrawal neuronal cell death, neurons maintained by MLK inhibitors require additional activation of the phosphatidylinositol 3-kinase (PI3-kinase) pathway for survival. [2,3]

In sympathetic neurons deprived of nerve growth factor, MLK inhibitors induce TrkA receptor over-expression. [3] Increases in TrkA expression cause ligand-independent activation of the receptor and, consequently, the PI3-kinase-Akt-GSK(glycogen synthase kinase)-3 pathway. Inhibition of PI3-kinase negates the long-term cell survival and trophic effects of MLK inhibitors.

In contrast, MLK inhibitors only maintain short-term survival in trophic-deprived cerebellar granule neurons (CGNs) that express TrkB, another Trk-receptor family member. While MLK inhibitors increase TrkB expression, this does not produce sustained activation of TrkB and the downstream PI3-kinase-Akt-GSK-3 pathway. MLK inhibitors require the addition of exogenous ligand, brain-derived neurotrophic factor (BDNF), to activate the PI3-kinase pathway and maintain long-term survival. In these CGNs, BDNF activates TrkB to maintain short-term survival; the survival is transient because of activation-dependent TrkB receptor down-regulation. Again, both MLK inhibition and ligand-mediated TrkB activation are required for sustained survival.

Therefore, as midbrain dopaminergic neurons express TrkB receptors, we hypothesize that inhibition of MLK and the JNK pathway will not maintain survival but will require the addition of BDNF to activate the PI3-kinase pathway. Perhaps this is the reason why MLK inhibitors failed to retard disease progression in PRECEPT.

An alternative to providing BDNF to TrkB-expressing neurons treated with MLK inhibitors is the inhibition of GSK-3, the downstream effector of PI3-kinase. The addition of GSK-3 inhibitors to MLK inhibitor-maintained CGNs is able to sustain long-term neuronal survival. [4] Perhaps with the potential availability of GSK-3 inhibitors, this is a worthy and testable hypothesis for dopaminergic and possibly other neurons.

References

1. The Parkinson Study Group PRECEPT Investigators. Mixed lineage kinase inhibitor CEP-1347 fails to delay disability in early Parkinson disease. Neurology 2007;69:1480-1490.

2. Roux PP, Dorval G, Boudreau M, et al. K252a and CEP1347 are neuroprotective compounds that inhibit mixed-lineage kinase-3 and induce activation of Akt and ERK. J Biol Chem 2002;277:49473-49480.

3. Wang LH, Paden AJ, Johnson EM, Jr. Mixed-lineage kinase inhibitors require the activation of Trk receptors to maintain long-term neuronal trophism and survival. J Pharmacol Exp Ther 2005;312:1007-1019.

4. Enguita M, DeGregorio-Rocasolano N, Abad A, Trullas R. Glycogen synthase kinase 3 activity mediates neuronal pentraxin 1 expression and cell death induced by potassium deprivation in cerebellar granule cells. Mol Pharmacol 2005;67:1237-1246.

Disclosure: The authors report no conflicts of interest.

We thank Dr. Wang et al for their comments. The PRECEPT results are disappointing since now two 'anti-apoptotic' compounds, CEP-1347 and TCH346, directed against the mixed lineage kinase (MLK) family and glyceraldehyde phosphate dehydrogenase (GAPDH), respectively, proved ineffective in slowing Parkinson disease (PD). [5]

The hypothesis provided by Drs. Wang and Johnson to explain the failure of CEP-1347 is a rational extrapolation based on current knowledge. In several neuronal systems, additional trophic support has been required to maintain longer term survival of neurons treated with CEP-1347. Additionally, BDNF levels have recently been shown to be reduced in the postmortem PD brain. [6] If the additional BDNF trophic signal is required for long-term survival of dopaminergic neurons, replacement or activation of this pathway may be necessary to restore the normal supportive environment. It remains unclear when in the disease progression a reduction in BDNF levels occurs and whether there is a point at which the rescue of neurons is not possible.

In cellular systems, the hypothesis is testable as to whether additional trophic support is needed to maintain longer term survival for CEP-1347-treated dopaminergic neurons. But, what is the appropriate model for dopaminergic cell death in PD? Will the same support be required under different insults? Model systems used in non-clinical research may not truly reflect the PD process. Protection of dopaminergic neurons against an MPTP insult has been demonstrated with CEP-1347 and GSK inhibitors. [7] However, BDNF levels are not decreased in models of neurotoxin-induced dopaminergic neuron loss. [8] The possibility that multiple points of interception/support may be needed to afford protection may require considerable re-thinking of the experimental therapeutics of neurodegenerative diseases and how to design clinical trials to test drug combinations.

Controlled trials like PRECEPT are powerful tools to detect pre-specified effects, if indeed they occur, but they poorly inform about pharmacological or pathogenic mechanisms. In the long run, decision making for experimental treatments in clinical trials depends on accrued scientific evidence, which will only be as good as the model systems employed to study the disease of interest.

With the consent of our research participants, the PRECEPT clinical trial has been transformed into a non-interventional sequel study called PostCEPT. [1] In addition to providing needed biomarkers for PD, PostCEPT will help us examine the long-term clinical and imaging outcomes in our PRECEPT research participants.

References

5. Waldmeier P, Bozyczko-Coyne D, Williams M, Vaught JL. Recent clinical failures in Parkinson's disease with apoptosis inhibitors underline the need for a paradigm shift in drug discovery for neurodegenerative diseases. Biochem Pharmacol. 2006 Nov 15;72:1197-1206. Epub 2006 Aug 9.

6. Nagatsu T, Sawada M. Biochemistry of postmortem brains in Parkinson's disease: historical overview and future prospects. J Neural Transm Suppl. 2007;:113-120.

7. Wang W, Yang Y, Ying C, Li W, Ruan H, Zhu X, You Y, Han Y, Chen R, Wang Y, Li M. Inhibition of glycogen synthase kinase-3beta protects dopaminergic neurons from MPTP toxicity. Neuropharmacology. 2007 Jun;52:1678-84. Epub 2007 Apr 19.

8.Mocchetti I, Bachis A, Nosheny RL, Tanda G Brain-derived neurotrophic factor expression in the substantia nigra does not change after lesions of dopaminergic neurons. Neurotox Res. 2007 Sep;12:135-43.

Disclosures: Ira Shoulson and Anthony Lang, authors of this correspondence on behalf of the Parkinson Study Group (PSG), received grant support from the sponsors Cephalon, Inc. (Frazer, PA), and H. Lundbeck A/S (Copenhagen-Valby, Denmark) through their academic institutions, but they neither had equity interests in nor received any personal remuneration from the sponsoring companies since initiation of the study. Donna Bozyczko-Coyne is an employee of the sponsor Cephalon, Inc. The PSG maintained the database and carried out independent analysis of the data.