A therapeutic trial of milacemide in myoclonus and the stiff-person syndrome

Stiff-person syndrome (SPS) and anti-GAD-related CNS degenerations: protean additions to the autoimmune central neuropathies

Stiff Person Syndrome (SPS) is a rare autoimmune neurological disease attributable to autoantibodies to glutamic acid decarboxylase (anti-GAD) more usually associated with the islet beta cell destruction of autoimmune type 1 diabetes (T1D). SPS is characterized by interference in neurons with the synthesis/activity of the inhibitory neurotransmitter gamma amino butyric acid (GABA) resulting in the prototypic progressive spasmodic muscular rigidity of SPS, or diverse neurological syndromes, cerebellar ataxia, intractable epilepsy, myoclonus and several others. Remarkably, a single autoantibody, anti-GAD, can be common to widely different disease expressions, i.e. T1D and SPS. One explanation for these data is the differences in epitope engagement between the anti-GAD reactivity in SPS and T1D: in both diseases, anti-GAD antibody reactivity is predominantly to a conformational epitope region in the PLP- and C-terminal domains of the 65 kDa isoform but, additionally in SPS, there is reactivity to conformational epitope(s) on GAD67, and short linear epitopes in the C-terminal region and at the N-terminus of GAD65. Another explanation for disease expressions in SPS includes ready access of anti-GAD to antigen sites due to immune responsiveness within the CNS itself according to intrathecal anti-GAD-specific B cells and autoantibody. Closer study of the mysterious stiff-person syndrome should enhance the understanding of this disease itself, and autoimmunity in general.

Stiff-person Syndrome

Stiff-person syndrome (SPS) is a progressive neurologic disorder characterized by 1) stiffness that is prominent in axial muscles, with co-contraction of agonist and antagonist muscles; 2) sudden episodic spasms; and 3) absence of another disease that causes similar symptoms. The diagnosis of SPS is based on clinical grounds and requires a high degree of suspicion. The diagnosis is, however, aided by electromyography, which demonstrates motor unit firing at rest simultaneously from the agonist and antagonist muscles, and by high serum titers of antibodies against glutamic acid decarboxylase (GAD), the rate-limiting enzyme for the synthesis of gamma-aminobutyric acid (GABA), which is the brain's main inhibitory neurotransmitter. The reduced GABA level in the brain and cerebrospinal fluid explains the patients' stiffness and justifies the clinical improvement observed by drugs enhancing GABAergic transmission. The association of SPS with other autoimmune disorders or autoantibodies, the intrathecal GAD-specific immunoglobulin G antibody synthesis, and the suppression of GABA by the patient's antibodies supports the autoimmune nature of SPS and justifies the use of immunotherapies. At present, GABA-enhancing agents, such as benzodiazepines, valproate, vigabatrin, tiagabine, gabapentin, and baclofen, provide symptomatic relief. Plasmapheresis, steroids, and periodic intravenous immunoglobulin infusions provide additional and lasting benefit. In this article, the treatment options for patients with SPS are discussed based on the authors' experience and that of others. The beneficial effects from the first controlled study conducted in SPS using intravenous immunoglobulin are presented.

Clobazam, oxcarbazepine, tiagabine, topiramate, and other new antiepileptic drugs

Clinical investigators recently have studied at least 21 new antiepileptic drugs (AEDs) in people with epilepsy. This review briefly examines 15 of these new AEDs: clobazam (CLB), dezinamide, flunarizine (FNR), loreclezole, milacemide (MLM), MK-801, nafimidone, ORG-6370, oxcarbazepine (OCBZ), progabide (PGB), ralitoline, stiripentol, tiagabine (TGB), topiramate (TPM), and zonisamide (ZNS). CLB, PGB, and TGB represent agents that act on the GABA system, and MLM acts on the glycine system. MK-801 and ZNS (in part) are excitatory amino acid antagonists, and FNR is a calcium-channel antagonist. OCBZ is a keto analogue of carbamazepine, which is not metabolized to the epoxide and may have fewer side effects. The remaining agents are novel compounds with a variety of suspected mechanisms. TPM appears especially effective for intractable partial seizures but has a high incidence of cognitive side effects. None of these new AEDs is useful for all patients with inadequate seizure control or ongoing toxicity. The role of each will require further clinical study and experience.

Therapeutic trial of milacemide in patients with myoclonus and other intractable movement disorders

We performed a therapeutic trial with the glycine precursor, milacemide, on 10 patients with intractable movement disorders. Six had myoclonus of various etiologies and one each had progressive supranuclear palsy, Filipino X-linked dystonia with parkinsonism, painful legs and moving toes, and stiff-person syndrome. Milacemide was initiated at a dose of 2,400 mg/day, orally, and increased gradually to a maximum of 4,800 mg/day. No clear-cut observable improvement occurred. There were no serious adverse effects.

Antiepileptic drug pharmacokinetics and neuropharmacokinetics in individual rats by repetitive withdrawal of blood and cerebrospinal fluid: milacemide

1. The kinetics and metabolism of milacemide have been studied in an animal model which allows the simultaneous investigation of the temporal inter-relationships of drugs and metabolites in blood (pharmacokinetics) and cerebrospinal fluid (CSF, neuropharmacokinetics) in individual freely moving rats. 2. Milacemide dose-dependently increased CSF glycine and glycinamide (intermediary metabolite) concentrations. This confirms that milacemide is a CNS glycine prodrug. 3. Pretreatment with L-deprenyl (2 mg kg-1), a specific inhibitor of monoamine oxidase type B (MAO-B), almost completely prevented the formation of glycinamide and increased milacemide accumulation in CSF. Tmax and t1/2 were significantly increased and Cmax and AUC values were decreased for glycinamide compared to controls. Pretreatment with clorgyline (5 mg kg-1), a specific inhibitor of MAO-type A, only moderately decreased glycinamide Cmax and AUC values. 4. After milacemide administration (100, 200 and 400 mg kg-1, i.p.) serum and CSF milacemide concentrations rose linearly and dose-dependently. Serum glycinamide concentrations exhibited small dose-dependent rises but these were not linearly related. In contrast, CSF glycinamide concentrations rose linearly and dose-dependently with Cmax values 2.5, 3.2 and 4.1 times greater than the corresponding values for serum glycinamide after giving 100, 200 and 400 mg kg-1 respectively of milacemide. 5. Serum glycine concentrations were unaffected but CSF concentrations increased dose-dependently and these were significant at the higher milacemide doses (200 and 400 mg kg-1). Animals given 400 mg kg-1 milacemide had glycine values which were still significantly elevated 7 h later. 6. In conclusion, serum milacemide rapidly enters and equilibrates with the CNS compartment where it is metabolised primarily by MAO-B to glycinamide and finally to glycine. Metabolism in the peripheral compartment is negligible.

Effect of milacemide on extracellular and tissue concentrations of dopamine and 5-hydroxytryptamine in rat frontal cortex

1. Milacemide is a glycine prodrug which is both an inhibitor and a substrate for monoamine oxidase-type B (MAO-B) and also an inhibitor of MAO-type A (MAO-A). Its effects on dopamine and 5-hydroxytryptamine (5-HT) metabolism in rat frontal cortex tissue and dialysate were evaluated. 2. Dialysate dopamine concentrations increased linearly and dose-dependently after milacemide administration (100, 200, 400 mg kg-1, i.p.), peaking at 1 h. A concomitant dose-dependent decrease in dialysate 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) concentrations was observed but these changes were smaller (27% and 40% respectively) than the change in dopamine (125% after 400 mg kg-1 milacemide). 3. Dialysate 5-HT was significantly increased only at 1.5 h after giving milacemide 400 mg kg-1. Dialysate 5-hydroxyindoleacetic acid (5-HIAA) concentration was not affected. 4. Milacemide (400 mg kg-1) at 1.5 h post-administration significantly increased frontal cortex tissue concentrations of dopamine and 5-HT; the percentage increase in dopamine being about four times that of 5-HT. Metabolite concentrations, including 5-HIAA, decreased. Changes in tissue and dialysate dopamine, DOPAC and HVA were approximately proportionate to each other. 5. The results are explicable in terms of an inhibition by milacemide of MAO-A.