SYNTHESIS AND PHARMACOLOGICAL EVALUATION OF PRIMARY AMINO ACID DERIVATIVES ...

406 Pages·2011·8.96 MB·English

by  Amber King

SYNTHESIS AND PHARMACOLOGICAL EVALUATION OF
PRIMARY AMINO ACID DERIVATIVES (PAADs):
NOVEL NEUROLOGICAL AGENTS FOR THE TREATMENT OF
EPILEPSY AND NEUROPATHIC PAIN

Amber Marie King

A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in
partial fulfillment of the requirements for the degree of Doctor of Philosophy in
Pharmaceutical Sciences at the Eshelman School of Pharmacy (Division of Medicinal
Chemistry and Natural Products).

Chapel Hill
2010

Approved by:
Dr. Harold Kohn
Dr. J. Phillip Bowen
Dr. Stephen Frye
Dr. Rihe Liu
Dr. Robert Rosenberg
ABSTRACT
AMBER MARIE KING: Synthesis and Pharmacological Evaluation of
Primary Amino Acid Derivatives (PAADs):
Novel Neurological Agents for the Treatment of Epilepsy and Neuropathic Pain
(Under the direction of Dr. Harold Kohn)

Epilepsy and neuropathic pain (NP) are chronic neurological disorders that result
from dysregulations in neuronal function. Currently, there is a lack of adequate therapeutic
agents available to treat these disorders and the need remains to develop compounds that
possess a novel mechanism of action to address the shortcomings of current medications.
Recently, the role of voltage-gated sodium channels (VGSCs) has been implicated in the
pathophysiological mechanisms of NP, while their role in epilepsy has been known for some
time. The functionalized amino acid (FAA) (R)-lacosamide is an emerging antiepileptic drug
(AED) that has been shown to selectively promote VGSCs into the slow inactivated state
and has recently been approved by the EMEA and the US FDA under the trademark
Vimpat® for the adjuvant treatment of partial-onset seizures in adults. (R)-Lacosamide has
also demonstrated clinical efficacy in treating painful diabetic neuropathy, but has yet to gain
regulatory approval for this indication.
The pharmaceutical industry has made advances in developing peripheral nervous
system (PNS)-specific agents that target specific isoforms of VGSCs for the treatment of
NP. We combined the concept of PNS-selectivity with our knowledge of FAAs and proposed
that primary amino acid derivatives (PAADs) may selectively target PNS receptor sites,
thereby avoiding potential CNS side effects that makes adherence to pain therapy difficult.
Additionally, we examined the effect of PAADs on CNS function due to the excellent
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anticonvulsant activity of FAAs. We synthesized and evaluated over 50 PAADs in whole
animal models of epilepsy and NP, and developed a structure-activity relationship (SAR)
that defined the structural requirements for PAAD activity. The SAR revealed excellent
anticonvulsant activity and pain attenuation for a novel class of compounds, the C(2)-
hydrocarbon PAADs. Then, we synthesized over 40 additional PAADs to optimize
anticonvulsant activity and pain attenuation. From our optimization studies, we discovered
two PAADs that displayed superior anticonvulsant activity and may rival the therapeutic
capabilities of (R)-lacosamide. Finally, we evaluated the most active PAADs in a series of
binding and enzymatic assays but we did not reveal any new binding targets of therapeutic
relevance.
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To my youngest sisters, Hailey and Zoe Smith:
You can achieve any goal with confidence and perseverance.
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PREFACE

As a young child, I possessed the gift of gab and a sense of confidence that often
resulted in borderline inappropriate conservations with strangers when my parents dared to
venture into public with me. As I grew older, I retained those qualities but learned the value
of modesty, which can be misinterpreted as shyness. My family and friends know better.

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TABLE OF CONTENTS

LIST OF TABLES....................................................................................................................xi
LIST OF FIGURES................................................................................................................xiv
LIST OF SCHEMES...............................................................................................................xv
LIST OF ABBREVIATIONS..................................................................................................xvii

Chapter 1. Introduction .......................................................................................................... 1
1. Central nervous system disorders .............................................................................. 1
1.1. Epilepsy .............................................................................................................. 2
1.2. Neuropathic pain ................................................................................................ 3
1.3. Antiepileptic drugs .............................................................................................. 4
1.3.1. Established AEDs: 1800s–1978 .................................................................. 4
1.3.2. Recent AEDs: 1993–Early 2000s ................................................................ 5
1.3.3. Emerging AEDs: Early 2000s–2010............................................................. 7
1.3.3.1. Lacosamide ............................................................................................ 9
1.4. Pain attenuating drugs .......................................................................................12
2. Communication methods of the nervous system .......................................................14
2.1. Electrical transmission .......................................................................................14
2.1.1. The action potential ....................................................................................14
2.1.2. VGSCs .......................................................................................................17
2.1.2.1. Structural properties of VGSCs ..............................................................17
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2.1.2.2. Functional states of the VGSC ...............................................................19
2.1.2.3. The role of VGSCs in nociception ..........................................................22
2.2. Chemical transmission .......................................................................................23
3. Primary amino acid derivatives as a treatment for CNS disorders.............................25
CHAPTER 2. Defining the Structural Requirements for PAADs ........................................... 30
1. Introduction ...............................................................................................................30
2. Results and discussion .............................................................................................30
2.1. Choice of compounds: PAADs ...........................................................................31
2.1.1. C(2)-(CH ) X(R) PAADs ............................................................................31
2 n y
2.1.2. 6-Membered and 5-membered aromatic and heteroaromatic PAADs .........33
2.1.3. C(2)-Hydrocarbon PAADs ..........................................................................34
2.2. Synthesis ...........................................................................................................35
2.2.1. C(2)-(CH ) X(R) PAADs ............................................................................35
2 n y
2.2.1.1. C(3)-Hydroxy and C(3)-alkoxy PAADs ...................................................35
2.2.1.2. C(3)-Amino PAADs ................................................................................41
2.2.1.3. C(4)-Substituted PAADs ........................................................................51
2.2.2. C(2)-6-Membered and 5-membered aromatic and heteroaromatic PAADs .55
2.2.3. C(2)-Hydrocarbon PAADs ..........................................................................56
2.2.4. Assessment of PAAD optical purity ............................................................58
2.3. Pharmacological evaluation ...............................................................................58
2.3.1. C(2)-(CH ) X(R) PAADs ............................................................................61
2 n y
2.3.1.1. C(3)-Hydroxy and C(3)-alkoxy PAADs ...................................................61
2.3.1.2. C(3)-Amino PAADs ................................................................................65
2.3.1.3. C(4)-Substituted PAADs ........................................................................67
2.3.2. 6-Membered and 5-membered aromatic and heteroaromatic PAADs .........68
2.3.3. C(2)-Hydrocarbon PAADs ..........................................................................76
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2.3.4. C(2) Stereochemistry and pharmacological activity ....................................85
2.3.5. Comparison of the data acquired at UCB and the NINDS ASP ...................88
3. Conclusions ..............................................................................................................90
4. Experimental ............................................................................................................93
4.1. General methods ...............................................................................................93
4.2. Synthesis ...........................................................................................................96
4.3. Pharmacology.................................................................................................. 177
4.3.1. Pharmacological evaluation conducted at UCB Pharma ........................... 178
4.3.2. Pharmacological evaluation conducted at the NINDS ASP ....................... 180
CHAPTER 3. PAAD Optimization ...................................................................................... 182
1. Introduction ............................................................................................................. 182
2. Results and discussion ........................................................................................... 183
2.1. Choice of compounds: 4‟-N-Benzyl substituted PAADs ................................... 183
2.1.1. 4‟-N-Benzyl substituted C(3)-O-methoxy PAADs ...................................... 184
2.1.2. 4‟-N-Benzyl substituted C(2)-hydrocarbon PAADs .................................... 184
2.1.3. 4‟-Chimeric C(2)-hydrocarbon PAADs ...................................................... 185
2.2. Choice of compounds: N-Substituted 3-methylbutanamides ............................ 188
2.3. Choice of compounds: C(2)-isopropyl PAAD analogs ...................................... 190
2.3.1. C(2)-Isopropyl PAAD analogs (Sites A–C) ................................................ 191
2.3.2. N-Benzyl substituted regioisomeric PAADs (Site D) ................................. 192
2.3.3. N-Substituted C(2)-isopropyl PAADs (Site E) ........................................... 192
2.3.4. α-Substituted N-benzylbutanamides (Site F)............................................. 192
2.4. Synthesis ......................................................................................................... 193
2.4.1. 4‟-N-Benzyl substituted C(2)-hydrocarbon PAADs .................................... 193
2.4.2. 4‟-Chimeric C(2)-hydrocarbon PAADs ...................................................... 194
2.4.3. N-Benzyl N’,N’-dimethylamino-3-methylbutanamide ................................. 199
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2.4.4. C(2)-Isopropyl PAAD analogs ................................................................... 199
2.5. Pharmacological evaluation ............................................................................. 206
2.5.1. 4‟-N-Benzyl substituted PAADs ................................................................ 207
2.5.1.1. 4‟-N-Benzyl substituted C(3)-O-methoxy PAADs ................................. 207
2.5.1.2. 4‟-N-Benzyl substituted C(2)-hydrocarbon PAADs ............................... 211
2.5.1.3. 4‟-Chimeric C(2)-hydrocarbon PAADs .................................................. 218
2.5.2. N-Substituted 3-methylbutanamides ......................................................... 226
2.5.3. C(2)-Isopropyl PAAD analogs ................................................................... 228
2.5.4. Comparison of data acquired at UCB and the NINDS ASP ....................... 237
3. Conclusions ............................................................................................................ 239
4. Experimental .......................................................................................................... 241
4.1. General methods ............................................................................................. 241
4.2. Synthesis ......................................................................................................... 243
4.3. Pharmacology.................................................................................................. 310
CHAPTER 4. Exploring the Mechanism of Action of PAADs ............................................. 312
1. Introduction ............................................................................................................. 312
2. Results and discussion ........................................................................................... 315
2.1. In vitro binding assays and functional receptor screens ................................... 315
2.1.1. C(3)-O-Methoxy-based PAADs ................................................................. 317
2.1.2. N-Benzyl safinamide-based PAADs.......................................................... 322
2.1.3. C(2)-Hydrocarbon-based PAADs .............................................................. 330
2.2. In vitro MAO enzymatic assay ........................................................................ 340
B
3. Conclusions ............................................................................................................ 341
4. Experimental .......................................................................................................... 342
4.1. Primary radioligand receptor screen ................................................................ 342
4.2. Secondary radioligand receptor screen............................................................ 344
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4.3. Functional receptor screen .............................................................................. 345
4.4. MAO enzymatic assay ................................................................................... 347
B
CHAPTER 5. General Conclusions and Future Directions................................................. 348
1. Conclusions ............................................................................................................ 348
2. Future directions ..................................................................................................... 349
2.1. Expanding the SAR of C(2)-hydrocarbon PAADs ............................................ 350
2.2. Prolonging the duration of action of C(2)-hydrocarbon PAADs ........................ 351
2.3. Electrophysiology studies ................................................................................ 352
2.4. Metabolism studies .......................................................................................... 354
2.5. Toxicity studies ................................................................................................ 356
REFERENCES.....................................................................................................................357
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