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Metanx in Type 2 Diabetes with Peripheral Neuropathy: A Randomized Trial

Published:December 06, 2012DOI:https://doi.org/10.1016/j.amjmed.2012.06.022

      Abstract

      Purpose

      To determine whether a combination of L-methylfolate, methylcobalamin, and pyridoxal-5′-phosphate (LMF-MC-PLP [Metanx; Pamlab LLC, Covington, La]) improves sensory neuropathy.

      Research Design and Methods

      This multicenter, randomized, double-blind, placebo-controlled trial involved 214 patients with type 2 diabetes and neuropathy (baseline vibration perception threshold [VPT]: 25-45 volts), who were randomly assigned to 24 weeks of treatment with either L-methylfolate calcium 3 mg, methylcobalamin 2 mg, and pyridoxal-5′-phosphate 35 mg or placebo. The primary end point was effect on VPT. Secondary end points included Neuropathy Total Symptom Score (NTSS-6) and Short Form 36 (SF-36), as well as plasma levels of folate, vitamins B6 and B12, methylmalonic acid (MMA), and homocysteine.

      Results

      There was no significant effect on VPT. However, patients receiving LMF-MC-PLP consistently reported symptomatic relief, with clinically significant improvement in NTSS-6 scores at week 16 (P=.013 vs placebo) and week 24 (P=.033). Improvement in NTSS scores was related to baseline MMA and inversely related to baseline PLP and metformin use. Quality-of-life measures also improved. Homocysteine decreased by 2.7±3.0 μmol/L with LMF-MC-PLP versus an increase of 0.5±2.4 μmol/L with placebo (P=.0001). Adverse events were infrequent, with no single event occurring in ≥2% of subjects.

      Conclusions

      LMF-MC-PLP appears to be a safe and effective therapy for alleviation of peripheral neuropathy symptoms, at least in the short term. Additional long-term studies should be conducted, as the trial duration may have been too short to show an effect on VPT. In addition, further research on the effects in patients with cobalamin deficiency would be useful.

      Keywords

      SEE RELATED EDITORIAL p. 95
      Diabetic peripheral neuropathy results from prolonged hyperglycemia through multiple complex mechanisms and causes debilitating neuropathic pain or loss of sensation in the extremities.
      • Figueroa-Romero C.
      • Sadidi M.
      • Feldman E.L.
      Mechanisms of disease: the oxidative stress theory of diabetic neuropathy.
      • Charles M.
      • Ejskjaer N.
      • Witte D.R.
      • Borch-Johnsen K.
      • Lauritzen T.
      • Sandbaek A.
      Prevalence of neuropathy and peripheral arterial disease and the impact of treatment in people with screen-detected type 2 diabetes: The ADDITION-Denmark study.
      Controlling hyperglycemia may prevent this condition, but treatment options are limited. Duloxetine and pregabalin are approved in the US for treatment of painful neuropathy but do not address the underlying pathology of the disease or improve sensation. The magnitude of the problem and the paucity of treatments call out for alternative approaches.
      • Approximately 35% of patients with diabetes have peripheral neuropathy, leading to debilitating pain or sensation loss and possible amputation.
      • Available treatments carry a high risk of adverse effects and do not address the underlying pathology of diabetic peripheral neuropathy.
      • Vitamin B deficiency is common in diabetes, particularly with metformin treatment.
      • A specific vitamin B formulation may represent a safe approach for diabetic peripheral neuropathy symptom relief.
      One possible novel tactic is to use a combination of L-methylfolate, methylcobalamin, and pyridoxal-5′-phosphate, the biologically active and immediately bioavailable forms of folate, vitamin B12, and vitamin B6, respectively. Deficiencies of vitamins B12 and B6 may contribute to neurologic deficits, and folate improves vascular function.
      • van Etten R.W.
      • de Koning E.J.
      • Verhaar M.C.
      • Gaillard C.A.
      • Rabelink T.J.
      Impaired NO-dependent vasodilation in patients with Type II (non-insulin-dependent) diabetes mellitus is restored by acute administration of folate.
      • Yaqub B.A.
      • Siddique A.
      • Sulimani R.
      Effects of methylcobalamin on diabetic neuropathy.
      • Watanabe T.
      • Kaji R.
      • Oka N.
      • Bara W.
      • Kimura J.
      Ultra-high dose methylcobalamin promotes nerve regeneration in experimental acrylamide neuropathy.
      • Okada K.
      • Tanaka H.
      • Temporin K.
      • et al.
      Methylcobalamin increases Erk1/2 and Akt activities through the methylation cycle and promotes nerve regeneration in a rat sciatic nerve injury model.
      • Wile D.J.
      • Toth C.
      Association of metformin, elevated homocysteine, and methylmalonic acid levels and clinically worsened diabetic peripheral neuropathy.
      • Solomon L.R.
      Diabetes as a cause of clinically significant functional cobalamin deficiency.
      • Bell D.S.
      Metformin-induced vitamin B12 deficiency presenting as a peripheral neuropathy.
      • McCann V.J.
      • Davis R.E.
      Serum pyridoxal concentrations in patients with diabetic neuropathy.
      Based on these observations, this study was designed to determine whether the medical food L-methylfolate calcium 3 mg, methylcobalamin 2 mg, and pyridoxal-5′-phosphate 35 mg (LMF-MC-PLP [Metanx; Pamlab LLC, Covington, La]) improves sensory neuropathy in patients with type 2 diabetes.

      Methods

      Patients aged 25 to 80 years with type 2 diabetes and neuropathy (baseline vibration perception threshold [VPT]: 25-45 volts at hallux on either leg) were recruited at 6 research clinics and hospitals throughout the US. Patients were randomly assigned to treatment with LMF-MC-PLP or placebo for 24 weeks. Opiates were not permitted, but other neuropathy medications (eg, pregabalin, gabapentin, duloxetine) could be used as long as therapies had been initiated for painful diabetic neuropathy more than 2 months before screening and doses were kept constant during the study. Dose modifications of current antidiabetic, lipid, and blood pressure medications were allowed at the discretion of the investigator, but no new treatments could be started during the trial or within the 2 months before screening. The protocol was approved by the institutional review board at each site, and all participants gave informed written consent before participation. The trial was conducted from June 2008 to May 2010.
      Exclusion criteria included peripheral vascular disease (palpable pedal pulse in both feet, no intermittent claudication, no history of lower extremity vascular bypass surgery or angioplasty); amputation or ulceration within 2 years before screening; Charcot neuroarthropathy; previous surgery to spine or lower extremity with residual pain or impaired mobility; severe arthritis causing pain upon walking; A1C >9% at screening; blood pressure >160/90 mm Hg or uncontrolled asthma or shortness of breath in the 2 months before screening; advanced renal disease (serum creatinine >2.5 times the upper limit of normal); pregnant or nursing; and history of alcohol or drug abuse within the past 3 years. The following additional restrictions were placed on supplement and medication usage: no α-lipoic acid or B12 injection within 2 months before screening; no more than 10 mg of B6 or 800 μg of folate within 2 months before screening; maximum dose of any anticonvulsant and no current treatment with systemic steroids, immunosuppressives, or radiotherapy.
      Randomization schedules were generated 1:1 separately for each of the 6 sites, with 50% assignment to either LMF-MC-PLP or placebo using a computer-generated randomization number list. The study medication and placebo were identical in appearance and were administered as 1 tablet taken twice daily.
      Outcomes were assessed at baseline and weeks 8, 16, and 24, and all assessment tools were used consistently across all sites and study populations. The primary end point of the study was VPT as measured by the VPT Meter (Diabetica Solutions, San Antonio, Tex) on the great toe of each foot. Secondary end points included neuropathic symptoms as evaluated by a modified 6-item Neuropathy Total Symptom Score (NTSS-6) and disability as measured by the Neuropathy Disability Score (NDS), as well as plasma levels of folate and its active form, 5-methyltetrahydrofolate (5-MTHF), PLP, vitamin B12 and its metabolite methylmalonic acid (MMA), and homocysteine. Additional secondary end points included health-related quality of life as determined by the Medical Outcomes Study Short-Form 36-Item Health Survey (SF-36) and participants' lower-extremity pain perception measured using a 10-point visual analog scale. Exploratory end points included plasma levels of high-sensitivity C-reactive protein, interleukin 6, malondialdehyde, potential antioxidant, tumor necrosis factor α, 3-nitrotyrosine, S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH). Depression also was evaluated as an exploratory outcome using the Hospital Anxiety and Depression Scale (HADS) question inventory.
      All adverse events were collected and reported using an Adverse Event Case Report Form. They included any undesirable sign, symptom, or medical condition occurring after starting the study intervention (ie, LMF-MC-PLP or placebo), including those considered unrelated to the intervention. Serious adverse events were identified as those that are fatal or life-threatening or result in hospitalization, significant disability, birth defects, or medical or surgical intervention.

      Statistical Analysis

      The sample size of 216 subjects was calculated based on an analysis of variance model, assuming an SD of 16 for the primary end point of change in VPT, with a significance level of 0.05, and assuming a drop-out rate of 20%, such that 90 patients per arm would complete the trial. For the purposes of the statistical analyses, assessments of VPT, visual analog scale, HADS, and the SF-36 had a normal distribution, while the NDS and NTSS-6 had a multinomial distribution. Overall and by subgroups, descriptive statistics such as means, SDs for continuous measurements, and frequencies and percentages for categorical measurements were summarized in tables. Fisher's exact test and chi-squared analysis were utilized to evaluate the differences between the 2 groups for categorical variables such as race and sex; Student's t test and analysis of variance were used to evaluate the difference between the 2 groups for continuous variables. Generalized linear mixed models were used to assess baseline for continuous and discrete outcomes, and generalized linear mixed models differing with respect to their fixed effects, random effects, and residual covariance matrix were examined. Type III tests were used to test the null hypothesis that a specific fixed effect equals zero, and Wald z-tests were used to test for significant random intercepts. MP (MPSOFTWARE ApS, Helsingør, Denmark) and SAS (SAS Institute Inc., Cary, NC) software were used to perform all analyses.

      Results

      Of 373 patients screened, 214 entered the study, and 200 completed it. Table 1 shows baseline characteristics for the study population. The mean age of participants was 62.6±8.9 years, with a mean duration of diabetes of 11.5±9.0 years and neuropathy of 6.1±6.3 years. The majority of participants were white men, but there were no differences between treatment groups in age, race, ethnicity, duration of diabetes or neuropathy, and baseline outcome measures. Mean A1C levels also were similar at baseline and remained stable throughout the study: at 24 weeks, A1C was 7.0% in both groups. At baseline, mean levels of vitamin B12, MMA, and homocysteine were all within normal limits. However, 12 patients in the LMF-MC-PLP and 18 in the placebo groups had below-normal baseline B12 levels, while homocysteine was elevated in 8 LMF-MC-PLP and 16 placebo patients, and MMA in 3 LMF-MC-PLP and 4 placebo patients, respectively.
      Table 1Baseline Characteristics
      LMF-MC-PLP (n=106)Placebo (n=108)P-Value
      Age, mean±SD (years)62.29±8.5462.95±9.17.5856
      Male sex, n (%)73 (68.9)75 (69.4)1.0000
      Race/ethnicity, n (%)
       Black/African American15 (14.2)9 (8.3).1801
       Other
      American Indian/Alaska Native, Asian, Native Hawaiian/Pacific Islander, or not defined.
      5 (4.7)2 (2.9)
       White86 (81.1)97 (89.8)
       Hispanic/Latino19 (17.9)18 (16.7).8577
      Diabetes duration, mean±SD (years)11.4±9.611.5±8.6.9366
      A1C, mean±SD (%)7.1±0.97.0±0.8.7408
      History of DPN, n (%)102 (96.2)101 (93.5).5381
      DPN duration, mean±SD (years)6.3±6.65.8±6.0.6276
      History of diabetic retinopathy, n (%)17 (16.0)19 (17.6).8555
      Medication usage
       Noninsulin agent monotherapy36 (33.9)32 (29.6)
        Metformin27 (25.5)21 (19.4)
        Sulfonylurea
      Glipizide, glimepiride, or glyburide.
      6 (5.7)5 (4.6)
        Glitazone
      Pioglitazone or rosiglitazone.
      1 (0.9)3 (2.8)
        DPP-4 inhibitor
      Sitagliptin.
      1 (0.9)2 (1.9)
        GLP-1 receptor agonist
      Exenatide.
      1 (0.9)1 (0.9)
       ≥2 Noninsulin agents31 (29.2)28 (25.9)
       Noninsulin agent(s)+insulin (of any type)24 (22.6)31 (28.7)
       Insulin alone (of any type)12 (11.3)14 (13.0)
      DPN=diabetic peripheral neuropathy; DPP-4=dipeptidyl peptidase; GLP-1=glucagon-like peptide 1; LMF-MC-PLP=L-methylfolate, methylcobalamin, and pyridoxal-5′-phosphate.
      low asterisk American Indian/Alaska Native, Asian, Native Hawaiian/Pacific Islander, or not defined.
      Glipizide, glimepiride, or glyburide.
      Pioglitazone or rosiglitazone.
      § Sitagliptin.
      Exenatide.
      Throughout the study, VPT, the primary outcome, did not differ significantly between the LMF-MC-PLP and placebo groups. Mean baseline VPT ranged from 32.16±13.00 to 33.88±14.24 volts across treatment groups and left and right great toes. By week 24, mean VPT (averaged across both toes) had decreased in both groups: −1.96±13.08 volts with LMF-MC-PLP and −3.27±10.32 volts with placebo. The treatment differences at each visit were not statistically significant.
      Table 2 shows the data for all secondary and exploratory end points. Neuropathy symptoms, as measured by changes in mean NTSS-6 scores, improved significantly in patients receiving LMF-MC-PLP: −0.90±1.42 at week 16 and −0.96±1.54 at week 24, compared with −0.40±1.72 (P=.013) and −0.53±1.69 (P=.033) in the placebo group at weeks 16 and 24, respectively (Figure 1). LMF-MC-PLP patients also experienced significantly less neuropathy-related disability as measured by the NDS at week 16 (−0.78±2.13 for LMF-MC-PLP vs. −0.18±2.24 for placebo; P=.027), although the difference was no longer statistically significant at week 24. Patients receiving LMF-MC-PLP also reported modest but significant improvement in quality-of-life measures. Scores on the mental component subscale of the SF-36 improved by almost 2 points with LMF-MC-PLP (1.99±8.57), compared with a decrease in the placebo group (−0.29±8.48; P=.031).
      Table 2Secondary and Exploratory End Points
      LMF-MC-PLPPlaceboP-Value
      Secondary
       NTSS-6 (Range: 0-6)
        Baseline3.73±1.793.45±2.05
        Change from BL, Week 16−0.90±1.42−0.40±1.72.013
        Change from BL, Week 24−0.96±1.54−0.53±1.69.033
       NDS (Range: 0-10)
        Baseline7.51±2.367.47±2.17
        Change from BL, Week 16−0.78±2.13−0.18±2.24.027
        Change from BL, Week 24−0.47±2.11−0.36±2.14NS
       Total homocysteine (μmol/L)
        Baseline9.71±4.299.47±3.90NS
        Change from BL, Week 16−2.70±2.900.58±2.58.0001
        Change from BL, Week 24−2.68±2.980.48±2.40.0001
       Total folate (nmol/L)
        Baseline42.19±9.9343.04±9.30NS
        Change from BL, Week 167.25±10.52−1.07±8.27.0001
        Change from BL, Week 247.53±10.42−2.75±8.26.0001
       5-MTHF (nmol/L)
        Baseline59.68±31.3260.73±31.35NS
        Change from BL, Week 16254.42±174.44−4.38±18.32<.0001
        Change from BL, Week 24229.70±163.42−2.13±25.15<.0001
       PLP (vitamin B6; nmol/L)
        Baseline70.15±65.1171.30±73.90NS
        Change from BL, Week 16213.17±152.615.55±54.34<.0001
        Change from BL, Week 24176.22±133.494.65±82.94<.0001
       Vitamin B12 (pmol/L)
        Baseline443.26±356.01408.25±245.67NS
        Change from BL, Week 162308.23±1729.8056.79±786.45<.0001
        Change from BL, Week 242241.96±1765.21−24.84±163.81<.0001
       MMA (nmol/L)
        Baseline186.16±120.11195.72±169.19NS
        Change from BL, Week 16−56.38±102.7213.89±125.21.0001
        Change from BL, Week 24−63.29±107.00−15.42±59.90.0008
       SF-36 PCS (Range: 0-100)
        Baseline40.47±10.3039.03±9.81
        Change from BL, Week 240.03±8.340.87±8.21NS
       SF-36 MCS (Range: 0-100)
        Baseline50.96±9.8752.66±8.39
        Change from BL, Week 241.99±8.57−0.29±8.48.0306
       VAS (Range: 0-10)
        Baseline3.26±2.773.25±2.76
        Change from BL, Week 24−0.27±2.28−0.03±2.61NS
      Exploratory
       HADS Depression (Range: 0-21)
        Baseline4.16±3.244.42±3.12
        Change from BL, Week 24−1.03±2.53−0.45±2.58.054
       hs-CRP (mg/L)
        Baseline6.43±5.987.44±7.23NS
        Change from BL, Week 24−0.71±4.050.13±4.15NS
       IL-6 (pg/mL)
        Baseline3.66±2.913.68±3.01NS
        Change from BL, Week 24−0.25±2.600.07±2.42NS
       MDA (μmol/L)
        Baseline1.52±0.701.48±0.62NS
        Change from BL, Week 240.05±0.55−0.02±0.57NS
       PAO (μmol/L)
        Baseline1094.52±230.831102.15±250.54NS
        Change from BL, Week 247.95±190.365.29±204.92NS
       TNF-α (pg/mL)
        Baseline1.69±1.252.01±2.72NS
        Change from BL, Week 240.03±0.76−0.04±0.54NS
      Additional assessments
       3-NT
        Baseline48.50±33.6245.65±34.68NS
        Change from BL, Week 242.04±21.13−2.61±25.21NS
       SAH
        Baseline35.93±20.7635.81±15.15NS
        Change from BL, Week 243.10±11.463.72±16.65NS
       SAM
        Baseline103.88±36.90105.22±37.64NS
        Change from BL, Week 246.12±30.69−2.70±30.61NS
       SAM-SAH ratio
        Baseline3.25±1.293.35±2.08NS
        Change from BL, Week 24−0.12±1.20−0.25±1.98NS
      Abbreviations: BL=baseline; HADS=Hospital Anxiety and Depression Scale; hs-CRP=high-sensitivity C-reactive protein; IL-6=interleukin 6; LMF-MC-PLP=L-methylfolate, methylcobalamin, and pyridoxal-5′-phosphate; MCS=mental component summary; MDA=malondialdehyde; MMA=methylmalonic acid; 5-MTHF=5-methyltetrahydrofolate; NDS=Neuropathy Disability Score; 3-NT=3-nitrotyrosine; NTSS-6=Neuropathy Total Symptom Score; PAO=potential antioxidant; PCS=physical component summary; PLP=pyridoxal-5′-phosphate; SAH=S-adenosylhomocysteine; SAM=S-adenosylmethionine; SF-36=Short Form 36; TNF-α=tumor necrosis factor α; VAS=visual analog scale.
      All values are means±SD.
      Figure thumbnail gr1
      Figure 1Change in mean Neuropathy Total Symptom Score (NTSS-6) from baseline to 24 weeks. This questionnaire includes 6 yes-or-no questions about neuropathic symptoms including various types of pain, tingling, and loss of sensation. Each “yes” answer is scored as 1 point. Lower scores signify improvement.
      As expected, plasma levels of folate and 5-MTHF, vitamin B12, and PLP increased significantly, while MMA levels decreased (Table 2). By week 24, homocysteine decreased significantly in the LMF-MC-PLP group by 2.68±2.98 μmol/L, compared with an increase of 0.48±2.40 μmol/L with placebo (P=.0001). There were no other statistically significant changes (Table 2).
      As shown in Table 3, improvement in NTSS-6 was significantly inversely associated with baseline PLP (P=.003). There also was a significant inverse association with SAM (P=.045) and a negative association with metformin use (P=.0215). The same model also showed that the change in quality of life was positively associated with MMA (P=.007).
      Table 3Results from Analyses of Biologic Parameters and Their Treatment Effects
      Generalized linear model equation: Y=μ+α1X1+α2X2+α3X3+… .+αkXk+ε, where Y is the outcome (response variable), μ is a constant, X is the covariant, α is the estimated effect of the covariate, and ε is any random error.
      CovariantEstimated Effect±SDP-Value
      NTSS-6
       Baseline PLP
      Model included baseline data and treatment effect.
      −0.005±0.002.003
       Increase in PLP at 24 weeks
      Model included change from baseline and treatment effect.
      0.004±0.001.003
       Increase in SAM at 24 weeks
      Model included change from baseline and treatment effect.
      0.011±0.005.045
       No metformin
      Model included change from baseline and treatment effect.
      −0.325±0.140.022
      SF-36 MCS
       Baseline MMA
      Model included baseline data and treatment effect.
      0.015±0.005.007
       Decrease in MMA at 24 weeks
      Model included change from baseline and treatment effect.
      −0.027±0.008.002
       Increase in SAM at 24 weeks
      Model included change from baseline and treatment effect.
      0.065±0.028.021
       Decrease in SAM-SAH ratio at 24 weeks
      Model included change from baseline and treatment effect.
      −1.042±0.485.033
      MMA=methylmalonic acid; NTSS-6=Neuropathy Total Symptom Score; PLP=pyridoxal-5′-phosphate; SAH=S-adenosylhomocysteine; SAM=S-adenosylmethionine; SF-36 MCS=Short Form 36 mental component summary.
      low asterisk Generalized linear model equation: Y=μ+α1X1+α2X2+α3X3+… .+αkXk+ε, where Y is the outcome (response variable), μ is a constant, X is the covariant, α is the estimated effect of the covariate, and ε is any random error.
      Model included baseline data and treatment effect.
      Model included change from baseline and treatment effect.
      Adverse events were infrequent, with no single event occurring in ≥2% of subjects. Table 4 shows rates of adverse events grouped by categories from the Medical Dictionary for Regulatory Activities,
      Maintenance and Support Services Organization
      Medical Dictionary for Regulatory Activities (MedDRA).
      where the combined total occurred in ≥2% of patients. There were 2 deaths during the trial, both in the placebo group. In addition, a transient ischemic attack occurred in 1 patient in the LMF-MC-PLP group, and a severe case of dyspnea occurred in 1 patient in the placebo group. Neither event was considered related to study intervention. There was 1 case of rash in the LMF-MC-PLP group that the investigator considered possibly related to study intervention. No patients withdrew from the study due to adverse events.
      Table 4AEs Occurring at a Frequency of ≥2% Per MedDRA Category*
      MedDRA Category, n (%)Metanx (n=106)Placebo (n=108)P-Value
      No AEs reported78 (73.6)79 (73.1)1.000
      Infections and infestations6 (5.7)6 (5.6)1.00
      Injury, poisoning, and procedural complications5 (4.7)4 (3.7).75
      General disorders and administration site conditions4 (3.8)3 (2.8).72
      Musculoskeletal and connective tissue disorders1 (0.9)6 (5.7).12
      Skin and subcutaneous tissue disorders3 (2.8)4 (3.7)1.00
      Surgical and medical procedures3 (2.8)1 (0.9).37
      Gastrointestinal disorders2 (1.9)1 (0.9).62
      Metabolism and nutrition disorders1 (0.9)2 (1.9)1.00
      AE=adverse event; MedDRA=Medical Dictionary for Regulatory Activities.

      Discussion

      The present study is the first randomized, placebo-controlled investigation to examine vitamin B in the form of LMF-MC-PLP. Over 24 weeks, VPT decreased in both treatment groups, with no significant between-group differences. In contrast, relative to placebo, LMF-MC-PLP significantly improved neuropathy symptoms at 16 and 24 weeks and disability at 16 weeks. Improvement in symptom scores was inversely associated with baseline PLP (P=.003) and positively associated with changes in PLP (P=.003) and SAM (P=.045). In addition, metformin use was associated with less improvement in NTSS-6 score (P=.0215). The mental component of the SF-36 improved significantly, suggesting that patients taking LMF-MC-PLP felt better. Adverse events were infrequent and mostly mild to moderate, occurred at similar rates in the 2 treatment groups, and did not lead to any withdrawals.
      The treatment failed to meet the primary end point of change in VPT. However, this measure may not be sensitive enough to detect changes in nerve function attributable to LMF-MC-PLP. This compound has been shown to improve sensory nerve conduction and fiber density in Zucker diabetic fatty rats.
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      The sensory symptoms of diabetic polyneuropathy are improved with alpha-lipoic acid: the SYDNEY trial.
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      • Reichel G.
      • Rett K.
      • et al.
      Treatment of diabetic polyneuropathy with the antioxidant thioctic acid (alpha-lipoic acid): a two year multicenter randomized double-blind placebo-controlled trial (ALADIN II) Alpha Lipoic Acid in Diabetic Neuropathy.
      • Ziegler D.
      • Low P.A.
      • Litchy W.J.
      • et al.
      Efficacy and safety of antioxidant treatment with α-lipoic acid over 4 years in diabetic polyneuropathy: the NATHAN 1 Trial.
      Currently, in the US, the only agents approved for the treatment of symptomatic diabetic peripheral neuropathy are duloxetine and pregabalin, which do not affect nerve conduction, have mechanisms of action unrelated to the pathophysiology of diabetic neuropathy, and are approved only for pain relief.
      • Ziegler D.
      Painful diabetic neuropathy: advantage of novel drugs over old drugs?.
      From the patient's perspective, symptom improvement may be the most important goal of neuropathy management, and the effect of LMF-MC-PLP on symptoms was both statistically and clinically significant. The NTSS-6 was chosen for this study because it measures sensation (2 items) as well as pain (4 items). The total score ranges from zero (no symptoms) to 20 (most severe for all symptoms), and the clinical significance of the change depends on baseline score and duration of treatment. For example, a small decrease from a low baseline of 5.7 over 1 year leads to meaningful changes in other parameters of neuropathy.
      • Bastyr 3rd, E.J.
      • Price K.L.
      • Bril V.
      Development and validity testing of the neuropathy total symptom score-6: questionnaire for the study of sensory symptoms of diabetic peripheral neuropathy.
      However, a larger decrease from a higher score (eg, 20 to 15) might leave patients with moderately severe symptoms. In the present study, patients were selected using VPT with no minimum symptom score required for inclusion. The resulting study population exhibited only mild symptoms on NTSS, with a mean baseline of 3.6. Thus, the patients in this study had 3-4 mild symptoms or 1-2 moderate symptoms at baseline. The observed 1-point decrease over the 6 months of our trial suggests that at least one mild symptom was completely eliminated or one moderate symptom could be classified as mild in the LMF-MC-PLP patients. The clinical significance of this effect is further supported by the improvement in the SF-36 mental component score.
      The exploratory analysis of the relationship between baseline and 24-week levels of the components of LMF-MC-PLP and their associated biologic factors (MMA, SAM, and SAH) suggests that improvements in the NTSS-6 and the SF-36 were related to baseline activity of the components of LMF-MC-PLP as well as changes that occurred on treatment. Whether these improvements would be maintained in the long term remains unclear.
      To date, controlled studies of polyneuropathy treatment with vitamin B have been small in scale, with varying results.
      • Yaqub B.A.
      • Siddique A.
      • Sulimani R.
      Effects of methylcobalamin on diabetic neuropathy.
      • Ang C.D.
      • Alviar M.J.
      • Dans A.L.
      • et al.
      Vitamin B for treating peripheral neuropathy.
      • Woelk H.
      • Lehrl S.
      • Bitsch R.
      • Kopcke W.
      Benfotiamine in treatment of alcoholic polyneuropathy: an 8-week randomized controlled study (BAP I Study).
      Nevertheless, the individual components of LMF-MC-PLP have been shown to counteract hyperglycemia-associated oxidative stress, considered a major cause of diabetic neuropathy.
      • Figueroa-Romero C.
      • Sadidi M.
      • Feldman E.L.
      Mechanisms of disease: the oxidative stress theory of diabetic neuropathy.
      L-methylfolate restores normal coupling of endothelial nitric oxide synthase, reversing the production of superoxide and oxidative-nitrative stress and restoring NO synthesis.
      • van Etten R.W.
      • de Koning E.J.
      • Verhaar M.C.
      • Gaillard C.A.
      • Rabelink T.J.
      Impaired NO-dependent vasodilation in patients with Type II (non-insulin-dependent) diabetes mellitus is restored by acute administration of folate.
      • Stroes E.S.
      • van Faassen E.E.
      • Yo M.
      • et al.
      Folic acid reverts dysfunction of endothelial nitric oxide synthase.
      These actions may lead to vasodilation and improved endothelial function in patients with diabetes.
      • Mangoni A.A.
      • Sherwood R.A.
      • Asonganyi B.
      • Swift C.G.
      • Thomas S.
      • Jackson S.H.
      Short-term oral folic acid supplementation enhances endothelial function in patients with type 2 diabetes.
      • Title L.M.
      • Ur E.
      • Giddens K.
      • McQueen M.J.
      • Nassar B.A.
      Folic acid improves endothelial dysfunction in type 2 diabetes—an effect independent of homocysteine-lowering.
      Methylcobalamin also neutralizes superoxide and peroxynitrite and restores normal glutathione levels and has been shown to improve neuropathy symptoms in both humans and animals.
      • Yaqub B.A.
      • Siddique A.
      • Sulimani R.
      Effects of methylcobalamin on diabetic neuropathy.
      • Watanabe T.
      • Kaji R.
      • Oka N.
      • Bara W.
      • Kimura J.
      Ultra-high dose methylcobalamin promotes nerve regeneration in experimental acrylamide neuropathy.
      • Okada K.
      • Tanaka H.
      • Temporin K.
      • et al.
      Methylcobalamin increases Erk1/2 and Akt activities through the methylation cycle and promotes nerve regeneration in a rat sciatic nerve injury model.
      • Solomon L.R.
      Diabetes as a cause of clinically significant functional cobalamin deficiency.
      • Ang C.D.
      • Alviar M.J.
      • Dans A.L.
      • et al.
      Vitamin B for treating peripheral neuropathy.
      • Wheatley C.
      The return of the Scarlet Pimpernel: cobalamin in inflammation II—cobalamins can both selectively promote all three nitric oxide synthases (NOS), particularly iNOS and eNOS, and, as needed, selectively inhibit iNOS and nNOS.
      Finally, pyroxidal may prevent formation of advanced glycation end products (AGEs), possibly via chelation of transition metals.
      • Price D.L.
      • Rhett P.M.
      • Thorpe S.R.
      • Baynes J.W.
      Chelating activity of advanced glycation end-product inhibitors.
      • Stitt A.
      • Gardiner T.A.
      • Alderson N.L.
      • et al.
      The AGE inhibitor pyridoxamine inhibits development of retinopathy in experimental diabetes.
      AGE inhibition by pyroxidal has been shown to reduce microvascular disease in animal models, although the effects in humans are not well defined.
      • McCann V.J.
      • Davis R.E.
      Serum pyridoxal concentrations in patients with diabetic neuropathy.
      • Nakamura S.
      • Li H.
      • Adijiang A.
      • Pischetsrieder M.
      • Niwa T.
      Pyridoxal phosphate prevents progression of diabetic nephropathy.
      • Levin E.R.
      • Hanscom T.A.
      • Fisher M.
      • et al.
      The influence of pyridoxine in diabetic peripheral neuropathy.
      In the present study, however, there was a significant inverse association between baseline pyridoxal levels and NTSS-6 scores, suggesting a possible benefit of pyridoxal supplementation.
      Mean vitamin B12 and MMA levels were within normal limits at baseline, and neither measure was significantly correlated with the NTSS-6 score at 24 weeks. Due to lack of power, we cannot determine whether methylcobalamin supplementation significantly affected the NTSS-6 scores of the patients with below-normal B12 levels. However, the negative association between NTSS-6 scores and metformin use suggests that methylcobalamin may have corrected a B12 deficiency. Such deficiency can be undetectable by measurement of serum values, and the use of elevated homocysteine and MMA as surrogate measures for vitamin B12 is increasingly recommended.
      • Saperstein D.S.
      • Wolfe G.I.
      • Gronseth G.S.
      • et al.
      Challenges in the identification of cobalamin-deficiency polyneuropathy.
      • Savage D.G.
      • Lindenbaum J.
      • Stabler S.P.
      • Allen R.H.
      Sensitivity of serum methylmalonic acid and total homocysteine determinations for diagnosing cobalamin and folate deficiencies.
      • Yetley E.A.
      • Pfeiffer C.M.
      • Phinney K.W.
      • et al.
      Biomarkers of vitamin B-12 status in NHANES: a roundtable summary.
      Metformin interferes with the absorption of cobalamin, which can lead to progressive deterioration of nerve tissue frequently misdiagnosed as diabetic peripheral neuropathy.
      • Wile D.J.
      • Toth C.
      Association of metformin, elevated homocysteine, and methylmalonic acid levels and clinically worsened diabetic peripheral neuropathy.
      • Solomon L.R.
      Diabetes as a cause of clinically significant functional cobalamin deficiency.
      • Bell D.S.
      Metformin-induced vitamin B12 deficiency presenting as a peripheral neuropathy.
      • Saperstein D.S.
      • Wolfe G.I.
      • Gronseth G.S.
      • et al.
      Challenges in the identification of cobalamin-deficiency polyneuropathy.
      Frank cobalamin deficiency increases with age in nondiabetic individuals, while functional deficiency (ie, normal cobalamin but elevated MMA levels) is increased in patients with type 2 diabetes (regardless of metformin use) and associated with a tripling of neuropathy frequency.
      • Solomon L.R.
      Diabetes as a cause of clinically significant functional cobalamin deficiency.
      Homocysteine, which may contribute to oxidative stress
      • Topal G.
      • Brunet A.
      • Millanvoye E.
      • et al.
      Homocysteine induces oxidative stress by uncoupling of NO synthase activity through reduction of tetrahydrobiopterin.
      and is a useful marker of vascular risk,
      • Title L.M.
      • Ur E.
      • Giddens K.
      • McQueen M.J.
      • Nassar B.A.
      Folic acid improves endothelial dysfunction in type 2 diabetes—an effect independent of homocysteine-lowering.
      • Clarke R.
      • Halsey J.
      • Lewington S.
      • et al.
      Effects of lowering homocysteine levels with B vitamins on cardiovascular disease, cancer, and cause-specific mortality: Meta-analysis of 8 randomized trials involving 37 485 individuals.
      is metabolized by both folate and cobalamin. Whether the significant decrease in homocysteine observed in our study contributed to symptom improvement cannot be determined from our results.
      This study has several limitations. First, the trial duration may not have been long enough to demonstrate an effect on VPT. Second, patients with possible cobalamin deficiency were not excluded—a more robust treatment response in such patients could have affected the results. Third, because the NTSS-6 is a composite tool, it was not possible to determine whether sensation, pain, or both improved during our trial. Future studies of LMF-MC-PP using either duloxetine or pregabalin as an active comparator may help clarify the compound's effects on pain.
      In summary, this study of LMF-MC-PLP showed an encouraging and statistically significant impact on neuropathy symptoms and quality-of-life measurements in patients with type 2 diabetes and peripheral neuropathy. The rate of adverse events was low and comparable with placebo. These findings support the use of LMF-MC-PLP as a safe approach for short-term alleviation of diabetic neuropathy symptoms, although its impact on long-term outcomes is not known. Additional studies are needed to further define these effects.

      Acknowledgements

      The authors thank Amanda Justice for assistance in preparation of the manuscript and Matthew Williams, BSc, and Erland Arning, PhD (Baylor Research Institute), for technical assistance with the biochemical analysis. This study was funded and conducted by Pamlab LLC.

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