"Lest we forget you - methylene blue ... "

Abstract

Methylene blue (MB), the first synthetic drug, has a 120-year-long history of diverse applications, both in medical treatments and as a staining reagent. In recent years there was a surge of interest in MB as an antimalarial agent and as a potential treatment of neurodegenerative disorders such as Alzheimer's disease (AD), possibly through its inhibition of the aggregation of tau protein. Here we review the history and medical applications of MB, with emphasis on recent developments.

Full text

Review
“Lest we forget you — methylene blue...”
R. Heiner Schirmer
a
, Heike Adler
a
, Marcus Pickhardt
b,c
, Eckhard Mandelkow
b,c,
*
a
Center of Biochemistry (BZH), University of Heidelberg, Heidelberg, Germany
b
Max-Planck-Unit for Structural Molecular Biology, Hamburg, Germany
c
DZNE, German Center for Neurodegenerative Diseases, Bonn, Germany
Received 20 September 2010; received in revised form 10 December 2010; accepted 21 December 2010
Abstract
Methylene blue (MB), the first synthetic drug, has a 120-year-long history of diverse applications, both in medical treatments and as a
staining reagent. In recent years there was a surge of interest in MB as an antimalarial agent and as a potential treatment of neurodegenerative
disorders such as Alzheimer’s disease (AD), possibly through its inhibition of the aggregation of tau protein. Here we review the history
and medical applications of MB, with emphasis on recent developments.
© 2011 Elsevier Inc. All rights reserved.
Keywords: Methylene blue; History; Medical application; Alzheimer’s disease; tau aggregation inhibitor; malaria therapy; prodrug of azure B
1. Introduction
The 2008 International Congress on Alzheimer’s Dis-
ease (ICAD) in Chicago disappointed many hopes for
treatments of Alzheimer’s disease that aimed at reducing
the amyloid burden in the brains of patients (discussed
previously on the AlzForum, see www.alzforum.org, 30
July 2009). At the same time, new expectations were
raised by reports on treatments designed to reduce the
neurofibrillary pathology of tau protein. One prominent
advocate of this approach was Claude Wischik who pre-
sented data arguing that the compound methylene blue
(MB) could reduce the aggregation of tau and thereby
slow down the disease (Hattori et al., 2008; Wischik et
al., 1996, 2008). This prompted an intense public debate
on the pros and cons of the “blue wonder” (Gura, 2008).
However, it was often forgotten that methylene blue, the
first synthetic drug, had already a 120-year history in
several areas of medicine.
2. Biochemistry of MB
MB is a tricyclic phenothiazine drug (Wainwright and
Amaral, 2005). Under physiological conditions it is a
blue cation which undergoes a catalytic redox cycle: MB
is reduced by nicotinamide adenine dinucleotide phos-
phate (NADPH) or thioredoxin to give leucoMB, an
uncharged colorless compound. LeucoMB is then spon-
taneously reoxidized by O
2
(Fig. 1). The typical redox-
cycling of MB in vivo can be illustrated in vitro using the
famous blue bottle experiment: MB is visibly reduced by
glucose to give leucoMB and then, by opening the lid of
the bottle it is reoxidized by atmospheric O
2
: the color
comes back. After closing the lid, there is a lag phase and
then MB is reduced again. Analogous phenomena have
been observed by pathologists at autopsies (Tan and
Rodriguez, 2008; Warth et al., 2009). MB is excreted in
the urine as a mixture of MB, leucoMB and demethylated
metabolites, e.g., azure B and azure A (Gaudette and
Lodge, 2005). MB-containing urine is very clear and has,
* Corresponding author at: Max-Planck-Unit, Structural Molecular Biol-
ogy, c/o DESY, Notkestrasse 85, D-22607 Hamburg, Germany. Tel.: ⫹49
40 8998 2810; fax: ⫹49 40 8971 6822.
E-mail address: mand@mpasmb.desy.de (E. Mandelkow).
Neurobiology of Aging xx (2011) xxx
www.elsevier.com/locate/neuaging
0197-4580/$ – see front matter © 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.neurobiolaging.2010.12.012

of course, a green or blue color which disappears a few
days after the last administration of MB (Guttmann and
Ehrlich, 1891).
3. History of MB
MB was the very first fully synthetic drug used in med-
icine. In 1891 it was applied by Paul Guttmann and Paul
Ehrlich for the treatment of malaria, and this application has
recently been revived (Coulibaly et al., 2009; Färber et al.,
1998; Vennerstrom et al., 1995). The famous Giemsa solu-
tion for staining and characterizing malaria parasites and
blood cells contains MB, eosin A, and azure B as active
principles (Barcia, 2007; Fleischer, 2004). Numerous other
microscopic discoveries including the identification of My-
cobacterium tuberculosis by Robert Koch and the structural
organization of nerve tissues (Cajal, 1896; Ehrlich, 1886;
Garcia-Lopez et al., 2007) were based on the biochemical
properties of MB. Staining with MB was also the beginning
of modern drug research (Kristiansen, 1989): Paul Ehrlich
argued that if pathogens like bacteria and parasites are
preferentially stained by MB, then this staining might indi-
cate a specific harmful effect on the pathogen which could
be exploited for fighting disease. This explains why the
terms “drug” and “dye” were used synonymously until
World War I.
Malaria and methylene blue played a major role also in
World War II. In 1943, General Douglas MacArthur, com-
mander of the Allied Forces in the Southwest Pacific theater
stated: “This will be a long war, if for every division I have
facing the enemy, I must count on a second division in the
hospital with malaria, and a third division convalescing
from this debilitating disease.” Because of the blue urine
(“Even at the loo we see, we pee, navy blue”), MB was not
well liked among the soldiers (MacArthur, 1964; W. Peters,
personal communication).
Going back to the beginning of the twentieth century,
MB was used for a wide variety of medical and hygienic
indications (Clark et al., 1925). Among others, it was added
to the medication of psychiatric patients in order to study
their compliance which could be monitored by the observ-
able color of the urine. These studies led to the discovery
that MB has antidepressant and further positive psychotro-
pic effects (Bodoni, 1899; Ehrlich and Leppmann, 1890;
Harvey et al., 2010). Thus MB became the lead compound
for other drugs including chlorpromazine and the tricyclic
antidepressants. In 1925 W. Mansfield Clark, famous for the
introduction of the pH electrode and the oxygen electrode,
was a coauthor of an impressive 80-page review on the
application of MB in engineering, industrial chemistry, bi-
ology, and medicine. A remarkable aspect of this article is
the reference list of illustrious scientists including several
Nobel Prize winners — Santiago Ramon y Cajal, Robert
Koch, Paul Ehrlich, Alphonse Laveran, Otto Meyerhof, and
Heinrich Wieland — who contributed major papers on MB.
Thus MB is an example for the high value of observations
and articles that were published 100 years ago and are still
relevant today.
4. Current medical indications
By 2010, there are more than 11,000 entries for “meth-
ylene blue” in the biomedical library PubMed, not counting
the studies which had been published in the era not covered
by PubMed. Current indications for MB that are approved
by the US Food and Drug Administration (FDA) are enzy-
mopenic hereditary methemoglobinemia and acute acquired
methemoglobinemia, prevention of urinary tract infections
in elderly patients (Table 1), and intraoperative visualiza-
tion of nerves, nerve tissues, and endocrine glands as well as
of pathologic fistulae (O’Leary et al., 1968).
Of great practical importance is also the administration
of MB for the prevention and treatment of ifosfamid-in-
duced neurotoxicity in cancer patients (Kupfer et al., 1994).
Recommended doses are 3 to 6 times 50 mg/day intrave-
nously (i.v.) or orally (p.o.) as a treatment and 3 to 4 times
50 mg/day p.o. given for prophylaxis, starting 1 day before
ifosfamid-infusion and continuing still after oxazaphospho-
rine-treatment is finished (Pelgrims et al., 2000). Concern-
ing inborn enzymopenic methemoglobinemia (Table 1), the
treatment of the Blue People of Troublesome Creek in
Kentucky — and other persons worldwide — with the blue
drug MB was a visible success of knowledge-based medi-
cine (Cawein et al., 1964). The rationale is that MB can be
reduced to colorless leucoMB by red blood cell enzymes
and that leucoMB reduces the inactive methemoglobin to
give hemoglobin (Fig. 1). This conversion turns the bluish
tinge of the skin to a rosy complexion — in the early 1960s
Fig. 1. Methylene blue (MB) as a redox-cycling phenothiazine drug in
vivo. MB is spontaneously or enzymatically reduced by NADPH and the
resulting uncolored leucoMB is reoxidized by molecular oxygen (O
2
)orby
iron(III)-containing compounds like methemoglobin. Precious NADPH
and O
2
are wasted, and hydrogen peroxide is produced in each round of the
cycle. There are also pharmacologic activities of MB which do not depend
on its redox properties. Azure B is monodemethylated MB, that is one of
the4CH
3
groups shown in the formula is replaced by a hydrogen atom.
Azure A is the didemethylated form at the same position, converting the
N(CH
3
)
2
to an NH
2
-group.
2H. Schirmer et al. / Neurobiology of Aging xx (2011) xxx

the right issue for emerging color television. Furthermore,
topical MB is the treatment of choice for priapism (Van der
Horst et al., 2003), and for intractable pruritus ani (Mentes
et al., 2004; Sutherland et al., 2009; Wolloch and Dintsman,
1979). Recently, MB was introduced against acute cate-
cholamine-refractory vasoplegia and other forms of shock
(Shanmugam, 2005). The current interest in MB as an
antimalarial compound and a potential drug in Alzheimer’s
disease is described below.
As to the pharmacokinetics of MB, a typical daily dosage
is 200 mg MB given orally (Table 1). The apparent half-life
of MB in the human body is approximately 10 hours; the
bioavailability is ⬃73%. After the oral intake of 500 mg
MB the concentration in blood peaks at 19
␮
M; after i.v.
administration of 50 mg MB the corresponding value is
approximately 2.2
␮
M(Walter-Sack et al., 2009); the di-
verging data of Peter et al. (2000) and of other authors are
discussed in the same report.
The distribution among organs depends on the form of
administration. When MB is given to rats intravenously, it will
accumulate in the brain. MB can permeate the blood-brain
barrier in rats irrespective of the administration route — intra-
peritoneally (i.p.) (O’Leary et al., 1968), intraduodenally, or
i.v. (Peter et al., 2000; Walter-Sack et al., 2009). With
patients it should never be given intrathecally. Concerning
MB toxicology, a dose of 7 mg/kg given i.v. leads to severe
gastro-intestinal symptoms in adult persons. For sheep, the
LD50 was found to be 42 mg/kg body weight (Burrows,
1984).
There are several contraindications for MB administra-
tion. This applies, for instance, to patients taking serotonin
reuptake inhibitors (Khavandi et al., 2008; Ramsay et al.,
2007) and possibly to persons with certain types of hered-
itary glucose-6-phosphate dehydrogenase deficiency (G6PD
deficiency). This enzyme provides antioxidant-reducing
equivalents in the form of NADPH. G6PD deficiency in its
different manifestations affects more than 500 million per-
sons in the world and is thus the most common potentially
hazardous hereditary condition.
On the other hand, positive side effects of MB acting as
a tonic have also been observed; these effects are possibly
due to an enhancement of mitochondrial activity (Cardama-
tis, 1900; Harvey et al., 2010; Riha et al., 2005).
5. Is MB the prodrug of azure B (monodemethyl
MB)?
MB is metabolized yielding N-demethylated molecules
like azure B and azure A (see Fig. 1). These compounds
have pharmacological effects as well (Culo et al., 1991;
Warth et al., 2009). For azure B and possibly other de-
methylated metabolites of MB as the active agents, MB is a
prodrug. For biological efficiency it is probably of advan-
tage that, in contrast to oxidized MB, oxidized azure B can
assume a neutral quinoneimine form that readily diffuses
through membranes. As summarized in Table 2, azure B
may be responsible for pharmacological effects ascribed
bona fide to MB. In the past, the distinction between MB
effects and azure B effects was not made because it was not
realized that there are many in vitro and in vivo conditions
leading to the conversion of MB to azure B. The examples
of Table 2 suggest that in most therapy-relevant parameters,
azure B is superior to methylene blue; it is for instance a
better inhibitor of human glutathione reductase and related
enzymes. One not very conspicuous exception to this rule is
the effect on growth and transmission of the malarial para-
site P. falciparum (Table 2).
The results of Culo et al. (1991) are most impressive.
Among other things, they found that, in contrast to MB, azure
B was capable of protecting 10 out of 10 mice from experi-
mentally-induced endotoxic shock and that, in another series
of experiments, azure B was capable of decreasing the blood
serum level of tumor necrosis factor-alpha (TNF alpha,
cachexin) by a factor of 10. It is unfortunate that the dosages
of MB and azure B in the latter test series were different. In
comparative studies it should be remembered that the dose-
response curves for both MB and azure B can have unusual
shapes, indicating hormetic effects which means that at high
concentrations a drug can have much less activity than at
intermediate levels (Bruchey and Gonzales-Lima, 2008).
MB has been reported to be a selective inhibitor of nitric
oxide (NO) synthase and of soluble guanylate cyclase, 2
enzymes involved in nitric oxide-mediated vasodilation; for
this reason MB is used in catecholamine-refractory septic
shock. Not yet studied are the effects of azure B on these
enzymes in the NO-induced signaling pathway (Warth et
al., 2009). The issue of MB versus azure B must also be
Table 1
Dosage of MB in different clinical conditions
Therapeutic indication Dosage of methylene blue
Inherited methemoglobinemia 1 ⫻50–250 mg/day (for a lifetime) (Cawein et al., 1964)
Acute methemoglobinemia 1–2 ⫻1.3 mg/kg (i.v. over 20 minutes)
Ifosfamid-induced neurotoxicity 4 ⫻50 mg/day p.o. or i.v. (Pelgrims et al., 2000)
Prevention of urinary tract infections in elderly patients Orally 3 ⫻65 mg/day
Vasoplegic adrenaline-resistant shock 200 mg i.v. over 1 hour followed by infusion (0.25–2 mg/kg/hour) (Warth et al., 2009)
Alzheimer’s disease 3 ⫻60 mg/day (Rember™ according to Wischik et al., 2008)
Pediatric malaria 2 ⫻12 mg/kg p.o. for 3 days
Key: i.v., intravenous; MB, methylene blue; p.o., oral.
3H. Schirmer et al. / Neurobiology of Aging xx (2011) xxx

clarified for the effects of the phenothiazines on the aggre-
gation behavior of neurodegenerative filaments (see below
and Table 2).
To our knowledge, azure B has been administered only
in rodents (Culo et al., 1991) but not in humans. One of us
(R.H.S., 60 kg) experienced that 120 mg azure B dissolved
in 30 mL water taken orally led to sensations similar to MB
at the same dosage: immediate transient blue discoloration
of mucous membranes and teeth, as well as bitter taste
(burning but not unpleasant for an adult). Later the urine
turned green and then blue, the color intensity being max-
imal at 12 hours. The tonic invigorating effect described for
MB (Cardamatis, 1900; Harvey et al., 2010; Riha et al.,
2005) was experienced for azure B at an oral dosage of 2
mg/kg but not at a dosage of 0.5 mg/kg. These observations
are consistent with the fact that varying amounts of azure B
are present as a contamination in clinically used MB prep-
arations, which has remained unnoticed for more than 100
years.
6. Pleiotropism of MB
There is an amazing number of different molecular tar-
gets which have been identified at a molecular level for MB
and/or its demethylated metabolites like azure B. The most
prominent targets are NO synthases, guanylate cyclase, met-
hemoglobin, monoamino-oxidase A, acetylcholine es-
terases, and disulfide reductases such as glutathione reduc-
tase or dihydrolipoamide dehydrogenase (Buchholz et al.,
2008; Harvey et al., 2010; Juffermans et al., 2010). As to the
interactions with the flavin-dependent disulfide reductases,
MB is not only a (noncompetitive or uncompetitive) inhib-
itor but also a substrate (Figs. 1 and 2). MB is reduced by
the flavoenzyme and the resulting leucoMB is spontane-
ously oxidized by molecular oxygen (O
2
) to give toxic
reactive oxygen species (ROS) like superoxide or hydrogen
peroxide while MB is formed again. In this way MB is
available for the next cycle; it acts — in a functional unit
together with flavoenzymes and molecular oxygen — as a
recycling catalyst against infectious organisms (Fig. 1).
Apart from medical applications, this is the basis for using
MB as a disinfectant (Clark et al., 1925), for example as a
fungicide in aquariums (see, e.g., www.americanaquarium-
products.com).
Table 2
Comparison of methylene blue with its monodemethylated metabolite azure B
Compared property or effect MB Azure B References
Intracellular reduced form Color-free leucoMB Leuco AB Wainwright and Amaral (2005)
Extracellular oxidized form Dark blue cation Dark blue cation
Oxidized and deprotonated form Not possible Neutral quinoneimine
Catalytic efficiency as substrates of glutathione
reductase
4800 M
⫺1
s
⫺1
9200 M
⫺1
s
⫺1
Buchholz et al. (2008)
Inhibition of glutathione reductase K
i
⫽16
␮
MK
i
⫽5
␮
MBuchholz et al. (2008)
Crystal structure of the ligand-glutathione reductase
complex
Known at low
resolution
Solved at 2 Å
resolution
Schirmer et al. (2008); Fritz-
Wolf (2010, personal
communication)
Proportion in rat urine after giving MB 50% 50% Gaudette and Lodge (2005)
Proportion in human liver, kidney, heart and lung at
autopsy after previous MB administration
5% to 14% 86% to 95% Warth et al. (2009)
Protection of mice from lethal LPS/endotoxic shock Two out of 10
animals
Ten out of 10
animals
Culo et al. (1991)
Suppression of tumor necrosis factor-alpha level
(TNF-alpha, cachexin)
To 10% of control To 50% of control Culo et al. (1991)
Growth inhibition of transplanted tumors in mice No Yes Culo et al. (1991)
Inhibition of malarial parasite propagation in culture IC50 ⫽4 nM IC50 ⫽8 to 13 nM Vennerstrom et al. (1995)
Inhibition of A
␤
-peptide aggregation IC50 ⫽2.3
␮
M IC50 ⫽0.3
␮
MTaniguchi et al. (2005)
Inhibition of tau-protein aggregation IC50 ⫽1.9
␮
M IC50 ⫽1.9
␮
MTaniguchi et al. (2005)
Inhibition of tau-protein aggregation K
i
⫽3.4
␮
MK
i
⫽112 nM Wischik et al. (1996)
Key: AB, azure B; IC50, required drug concentration for 50% inhibition; LPS, lipopolysaccharide; MB, methylene blue.
Fig. 2. Crystal structure of the human dimeric enzyme glutathione reduc-
tase (GR) with bound tricyclic inhibitor compound. The 2 GR monomers
in the dimer are shown with different colors (cyan and green). A xanthene
structure is shown as a pink stick model. Methylene blue (MB) derivatives
and pyocyanin also bind to this site at the subunit-subunit interface
(Schirmer et al., 2008; Fritz-Wolf et al., personal communication). There is
probably an additional binding site for phenothiazines acting as reducible
substrates. Modified from Savvides and Andrew-Karplus (1996) and Sarma
et al. (2003).
4H. Schirmer et al. / Neurobiology of Aging xx (2011) xxx

7. MB as an analogue of the phenazine compound
pyocyanin
An explanation for the pleiotropism of MB could be that
it is a thioanalogue of the blue secondary metabolite pyo-
cyanin which acts as a signaling compound and a virulence
factor in bacteria (Dietrich et al., 2008). Secondary metab-
olites (Kossel, 1908) such as caffeine, acetyl salicylate
(ACC; aspirin) and most antibiotics have many effects in
different organisms but evolution has primed them never-
theless for biospecific interactions (Ahuja et al., 2008; Di-
etrich et al., 2006, 2008). Human-designed synthetic drugs,
in contrast, have not gone through this evolutionary training
unless they closely resemble natural compounds.
Recently the pharmacologically active selenoanalogue of
MB was synthesized by Herbert Zimmermann (Max Planck
Institute, Heidelberg, unpublished). He used a new method
whereby selenium — instead of the sulfur (see MB formula
in Fig. 1) — is introduced in the very last step of synthesis.
The isotopes of seleno-MB, especially the radioactive tracer
Se-75-MB, are of interest for systematic studies on the
pharmacokinetics and pharmacodynamics of seleno-MB
and seleno-azure B (Kühbacher et al., 2009; Kung and Blau,
1980). From a structural perspective, seleno-MB has the
additional advantage to allow accurate determination of
crystalline enzyme-MB complexes because Se atoms act as
anomalous diffraction centers for synchrotron X-ray diffrac-
tion (Hendrickson, 1991).
8. Methylene blue for falciparum malaria in children
The revival of MB as an antimalarial drug candidate
began in 1995 in 3 biochemical laboratories (Atamna et al.,
1996; Färber et al., 1998; Vennerstrom et al., 1995). The
major goal of this work is to develop an affordable, avail-
able, and accessible therapy for uncomplicated falciparum
malaria in children under 5 years of age in Africa. The
results of the clinical studies in Nouna, Burkina Faso (di-
rected by Olaf Müller, Boubacar Coulibaly, and Peter
Meissner), are promising. Methylene blue-based combina-
tion therapy is efficacious and safe even for children with
the African form of glucose-6-phosphate dehydrogenase
deficiency (G6PD deficiency A
minus
); the latter condition
affects ⬃15% of the male population in West Africa. As
shown in an anthropological study, MB-based therapies are
accepted by the communities in spite of the blue discolor-
ation; on the contrary, blue washable spots in clothes or
diapers indicate patient compliance to caregivers and health
workers (Coulibaly et al., 2009; Müller et al., 2008). The
blue color of the urine can also be used as an indicator that
the MB-containing drug combination has not been faked.
Drug faking is a most serious problem in many developing
countries (Müller et al., 2009).
Pharmacokinetics and bioavailability of MB given
orally have recently been reinvestigated (Akoachere et
al., 2005; Bountogo et al., 2010; Walter-Sack et al.,
2009); details are given in the legend of Table 1.MBis
active in vitro and in vivo not only against the malaria-
causing blood schizonts (Vennerstrom et al., 1995), but
also against the gametocytes of P. falciparum which are
responsible for disease transmission from patients to
mosquitoes (Buchholz et al., 2008; Coulibaly et al.,
2009). As a therapy, 2 oral doses of 12 mg MB/kg body
weight are administered for 3 days, which is in total 72
mg/kg (Zoungrana et al., 2008). Formulations that are
suitable for children, a sweet granulate or syrup of MB,
have recently been developed (Gut et al., 2008).
Olaf Müller and his team did not observe different
effects of MB when using different commercial sources
including the 0.1 g MB capsules prepared for us at the
University Pharmacy. Since 2007, however, we have had
difficulties in obtaining sufficient MB from pharmaceu-
tical companies for the clinical trials in West Africa. In
Germany, MB is no longer available even in pediatric
emergency rooms (Ludwig and Baethge, 2010). In addi-
tion, it was claimed that the available MB preparations
were not pure enough, the major contaminants being
heavy metals, azure B, and water. By contrast, we regard
the prevailing requirements of USP and EP (listed for
instance in www.provepharm.com/analysis.php)asap-
propriate. Taking heavy metal ions as examples, copper
and chromium are essential nutrients, and it is interesting
to compare their contents in a daily MB dose with their
contents in the ingredients of a standard meal. As a
conservative physician one is often concerned about the
overblown safety requirements of postmodern medicine
which too often prevent health- or even life-saving mea-
sures.
The explanations given for the shortage of MB were
contradictory; the most plausible one was that there are
ongoing market rearrangements and price reassessments.
Indeed, the price for MB as a raw material — which is now
Good Manufacturing Practice (GMP) validated — has re-
cently gone up by a factor of 100. Thus MB will probably
no longer serve as an affordable compound for treating
malaria as a disease of the poor. In this context it should also
be emphasized that MB used to be an ethical preparation
which implies that the drug is affordable and available
everywhere in sufficient dosages for patients who need it,
even when considering that the incidence of malaria ex-
ceeds 250 million cases per year. Our title “Lest we forget
you” (originally from a poem of Rudyard Kipling, 1887 and
later often used as a solemn formula on Remembrance
Days) was chosen because we regard MB as a drug that has
saved many lives and deserves our continuous respect like
other heroes. It is indeed debatable if development, produc-
tion, and distribution of drugs for malaria and other diseases
of the poor should be left to the pharmaceutical industry as
there is an enormous need but little demand for these drugs,
which means that market rules do not apply.
5H. Schirmer et al. / Neurobiology of Aging xx (2011) xxx

9. MB for slowing down neurodegenerative diseases?
For the treatment of Alzheimer’s disease different ap-
proaches are followed. A
␤
- or tau-based strategies focus on
the modulation at the protein level (e.g., inhibition of
proteolytic processing or of phosphorylation) or on the
inhibition of protein aggregates. Other potential drugs for
Alzheimer’s disease (e.g., Dimebon (Biotrend Chemicals),
NAP-peptide (NAPVSIPQ; Hölzel Diagnostika)) have dif-
ferent targets (acetylcholine esterase, mitochondrial respi-
ration, cytochrome c-oxidase, NMDA receptor, microtubule
stability) and thus may improve the viability of the affected
cells. The drug candidates which are presently tested in
clinical trials for Alzheimer’s disease were reviewed by
Neugroschl and Sano (2009, 2010).
During the past decade there has been a growing interest
in MB as a potential drug for Alzheimer’s disease. This was
based on the observations that MB can inhibit the aggrega-
tion of tau protein (Wischik et al., 1996) and of A
␤
peptides
in the low
␮
mol/L range (Chang et al., 2009; Necula et al.,
2007; Taniguchi et al., 2005). The potential role of MB as
an aggregation inhibitor of Tau can be measured by various
assays (Figs. 3 and 4). The reported IC50 values for MB
regarding the inhibition of Tau-aggregation show some
variation, e.g., 1.9
␮
M (see Table 2 in Taniguchi et al.,
2005) or approximately 3.5
␮
M(Chang et al., 2009, their
Fig. 4). Some of this may be due to different experimental
procedures (e.g., filter-based assays vs. fluorescence detec-
tion methods). Pelleting assays with polyacrylamide gel
electrophoresis as well as electron microscopy reflect not
necessarily the full composition of mixed protein aggregates
in a given sample because of incomplete separation or
absorption. On the other hand the filter method contains
little information about the quality (ordered or amorphous)
of the detected protein aggregates. Furthermore, the amount
of aggregated protein and the activity of aggregation inhib-
itors are strongly influenced by incubation parameters like
the incubation temperature, time, ionic strength and pH of
the buffer system (Jeganathan et al., 2008), nature and
amount of aggregation-inducing substance (e.g., heparin,
arachidonic acid) as well as the properties of the used
protein. For example, certain compounds have only 1 bind-
Fig. 3. Inhibition of Tau aggregation by methylene blue (MB) determined
by the filter assay. Tau protein (repeat domain, construct K19) was aggre-
gated in the presence of various concentrations of methylene blue. The
samples were filtered through a nitrocellulose membrane (Ø 0.45
␮
m) and
the amount of aggregates were detected via Western blotting. For details
see Pickhardt et al. (2007), and Bulic et al. (2010).
Fig. 4. Electron micrographs of tau filaments (repeat domain) after and before treatment with methylene blue.
6H. Schirmer et al. / Neurobiology of Aging xx (2011) xxx

ing site in the repeat domain but several in full-length tau,
which changes the apparent IC50 concentrations. In sum-
mary, these experimental assays may show differences in
detail but reflect similar trends (Figs. 3 and 4). The bottom
line of Table 2 shows that azure B is a more effective
aggregation inhibitor than MB. As a consequence, the pro-
portion of azure B present as an impurity of an MB batch
can influence the apparent inhibition constant.
MB received widespread attention following the report by
Wischik and colleagues at the International Conference on
Alzheimer’s Disease in Chicago in 2008 that the formulation
Rember
TM
had beneficial effects in clinical trials (Wischik et
al., 2008). This claim triggered a wave of comments in scien-
tific journals, blogs, and the public press (for comments by the
scientific community see www.alzforum.org). The aggrega-
tion inhibitory potential was shown in cell models not only
for tau and A
␤
, but for other aggregation-prone proteins,
notably TDP43 involved in frontotemporal dementias (Arai
et al., 2010; Yamashita et al., 2009). It should be noted that
even in the case of the dimeric enzyme glutathione reduc-
tase, MB or its demethylated metabolites, bind at the inter-
face of the subunits (Fig. 2).
As expected of MB’s pleiotropism, other modes of action
for MB in Alzheimer’s disease were proposed as well. A
protective role of MB in senescence and neurodegeneration
was postulated to operate via mitochondria and cytochrome
c oxidase and thought to be based on the cycling between
the oxidized and reduced forms of MB (Atamna and Kumar,
2010; Atamna et al., 2008). It is presently not clear whether
a possible therapeutic use of MB in the treatment of neu-
rodegenerative diseases is due to its tau aggregation inhib-
itory effect, the antioxidant activity by interaction with the
electron transport chain of mitochondria, or the binding and
modulation of other proteins.
Behavioral and memory studies with rats showed im-
provement with low doses of MB (a few mg/kg) without
other side effects (Riha et al., 2005). This is consistent with
the doses that were shown to have no adverse side effects in
humans (⬃300 mg/day, or ⬃4 mg/kg, Naylor et al., 1986).
The positive memory effects are assumed to be due to
improved brain glucose and oxygen utilization via stimula-
tion of mitochondrial cytochrome c oxidase. However, ad-
verse effects (e.g., on locomotion) became manifest at
higher doses.
Treatment of 3xTg-AD mice showed an improved clear-
ance of soluble A
␤
and increased chymotrypsin- and tryp-
sin-like activities of the proteasome, but no effect on tau
accumulation, phosphorylation or mislocalization (Medina
et al., 2010).
Mixed conclusions were drawn from studies on the in-
teraction between MB and the cellular chaperone system in
cell models. MB inhibits the ATPase of hsp70, and one of
the consequences is the reduced degradation of toxic poly-
Gln tracts and their increased accumulation in the cell
(Wang et al., 2010). On the other hand, the same type of
hsp70 inhibition is thought to selectively decrease the level
of tau protein, thus protecting the cell from toxic accumu-
lations of tau (Jinwal et al., 2009). In zebra fish models of
tauopathy, MB showed no effects, in contrast to analogous
zebra fish models expressing poly-Gln proteins; here aggre-
gation could be prevented by poly-Gln aggregation inhibi-
tors (van Bebber et al., 2010). In the case of rats, MB was
able to reverse the cognitive deficits induced by scopol-
amine and it acted synergistically with rivastigmine, an
acetyl cholinesterase inhibitor used in the treatment of Alz-
heimer’s disease (AD) (Deiana et al., 2009). This result may
be explained by the fact that MB acts as a cholinesterase
inhibitor as well (Pfaffendorf et al., 1997). Thus MB is
expected to interact with the effects of cholinesterase inhib-
itors presently used in the therapy of Alzheimer’s disease.
Further information on the physicochemical features and
molecular targets in the brain can be found in the reports of
Oz et al. (2009, 2011).
One caveat in the interpretation of drug studies is that
methylene blue shows a biphasic dose-response relationship
with beneficial effects only in an intermediate concentration
zone (hormetic effect). This was shown for brain cyto-
chrome oxidase activity and spontaneous locomotor activity
in the running wheel test (Bruchey and Gonzales-Lima,
2008). A similar behavior was described for cyanine com-
pounds on tau aggregation in tissue slices (Chang et al.,
2009).
Even though the reports on the effects of MB are mixed,
there is little doubt that many patients and their caregivers
who suffer now, are not willing to wait until MB in the form
of Rember
TM
might be registered as a drug for use against
Alzheimer’s disease in 2012 or later (especially since “dos-
age recommendations” for AD patients are traded on pop-
ular web sites). In order to get additional insights into the
efficacy of MB it would be interesting to retrospectively
study possible neurological effects of MB in patients who
have taken MB for the prevention of urinary tract infections
(Table 2). If the effects of MB and/or its metabolites on the
pathogenesis of AD can be confirmed, these compounds
could be administered prophylactically probably for de-
cades. This is illustrated by persons with hereditary methe-
moglobinemia who take MB for a lifetime in order to
prevent the accumulation of the pathological protein met-
hemoglobin in red blood cells (Cawein et al., 1964).
Disclosure statement
The authors declare no conflict of interest.
Acknowledgements
H.A. and R.H.S. are indebted to the DFG (SFB 544
“Control of tropical infectious diseases” subproject B2) and
to the Dream Action Award of DSM-Austria for continuous
generous support. E.M. acknowledges partial support by
7H. Schirmer et al. / Neurobiology of Aging xx (2011) xxx

grants from the BMBF (KNDD project) and the EU (Mem-
osad project). We thank Karin Fritz-Wolf (Max-Planck-
Institute for Medical Research, Heidelberg) for sharing her
knowledge of phenothiazine drug binding sites, Herbert
Zimmermann (of the same institute) for the de novo syn-
thesis of 99% pure selenomethylene blue, and Alexander
Marx (MPI Hamburg) for generating Figure 2.
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