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Normal
ly
Diseased
Elimination
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246
sosome
Breakdown
Recycled
lysosome
Substrate
accumulation
Tissue/organ damage
and
diesease symptoms
Figure9.1 Comparison of a healthy lysosome with a defective one of LSD patients: the
genetic mutation results in lysosome failure, hampering the breakdown and recycling of
macromolecules and leading to tissue and organ dysfunctions with consequent disease
symptoms.
Epidemiology and diagnosis of LSDs, as well as challenges for the screening of carriers, highrisk and newborn populations have been carefully reviewed by Wijburg
and coworkers[4].
Information about the incidence of LSDs is relatively limited. However, although
rare if taken individually (one case per 50 000–250 000individuals), they are common as a group, with an estimated incidence of 1in 5000–5500live births[5]. The
most common LSDs are Fabry, Gaucher, and Pompe diseases (PDs), with up to 2–2.5
cases per 100
000 individuals [6]. Moreover, despite a general low occurrence of
individual LSDs, the incidence may be higher in specific ethnicities, such as in the
case of Gaucher disease (GD), for which a frequency of 1 in 500 to 1in 1000 is
observed among Ashkenazi Jews[7].
LSDs related to enzyme deficiencies, which are the focus of this chapter, can be
subclassified according to the stored materials (see Table9.1):
Mucopolysaccharidoses (MPS) (such as MPS I, II, III e IVA)
12- Sphingolipidoses (such as Fabry, Niemann–Pick (NP) and GD, GM1 and
GM2gangliosidosis, and Krabbe disease (KD))
3- Glycogen Storage Disorders (namely, Pompe disease)
4- Glycoproteinoses (such as αMannosidosis and Fucosidosis)
Each disease can be in turn classified into different types based on symptoms,
affected organs, and age of onset, such as congenital or infantile (which usually
have the most severe presentation), lateinfantile, juvenile, and adult types.
Table9.1 Lysosomal storage disorders object ofthis chapter, classified according tothe
stored material: foreach disorder thename(s), thedeficient enzyme, and theapproved
therapy are listed.
Approved
Disease Deficient lysosomal enzyme
therapies
Mucopolysaccharidoses (MPS)
MPS I: Hurler syndrome αiduronidase HSCT, ERT
MPS II: Hunter syndrome Iduronate2sulfatase (IDS) ERT
MPS IIIA: Sanfilippo
Nsulfoglucosamine sulfohydrolase NONE
a
syndrome A
MPS IIIB: Sanfilippo
Nacetylαglucosaminidase (NAGLU) NONE
a
syndrome B
MPS IIIC (Sanfilippo
syndrome)
MPS IIID (Sanfilippo
Acetyl CoA glucosamine
Nacetyltransferase
Nacetylglucosamine6sulfatase NONE
NONE
a
a
syndrome)
MPS IVA: Morquio syndrome N‐acetylgalactosamine6sulfatase
ERT
(GALNS)
MPS IVB (Morquio B
syndrome)
b
MPS VI (Maroteaux–Lamy
βgalactosidase NONE
Arylsulfatase B ERT
a
syndrome)
MPS VII (Sly syndrome) βglucuronidase ERT
MPS IX Hyaluronidase NONE
a
Sphingolipidoses
Fabry disease αgalactosidase A ERT, PCT
Gaucher disease βglucocerebrosidase, also known as
ERT, SRT
acid βglucosidase (GCase)
Niemann–Pick Sphingomyelindegrading enzyme acid
ERT
sphingomyelinase and cholesterol
transport proteins
GM1gangliosidosis βgalactosidase NONE
GM2gagliosidosis: Tay–Sachs
βhexosaminidase NONE
a
a
disease and Sandhoff disease
Krabbe disease Galactosylceramidase HSCT
Glycogen storage disease
Pompe disease αglucosidase ERT
Glycoproteinoses
Fucosidosis αfucosidase NONE
a
αmannosidosis αmannosidase ERT
247
a) “NONE” means that only symptomatic and supportive therapies are provided to patients.
b) Being βgalactosidase the deficient enzyme in MPS IVB, this disorder will be discussed
together with GM1 (Sphingolipidoses section) instead of in the Mucopolysaccharidoses section.
ERT, enzyme replacement therapy; SRT, substrate reduction therapy; PCT, pharmacological
chaperone therapy.
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9.2 Available Treatments for LSDs: The Role of
Carbohydrate-Based Therapeutics
The promulgation of U.S. Orphan Drug Act (ODA) in 1983, followed by analogous
“orphan” legislation in other countries, undoubtedly stimulated the investments in
novel therapies for rare conditions, including LSDs, that otherwise would not be
profitable due to the small target population[8]. The consequently growing understanding of the pathophysiology of LSDs reached in the last decades, allowed to
identify various potential clinical intervention points, thus developing innovative
therapies, some of which passed all the steps from preclinical stage to regulatory
approval, and they were ultimately made available for patients. The most relevant
therapeutic approaches for LSDs are briefly described below.
The first treatment of an LSD, which showed any success was the hematopoietic
stem cell transplantation (HSCT) in a oneyearold MPS I patient, in 1981[9]. In
particular, the transplanted bone marrowderived cells with normal enzyme levels
were thought to donate enzymes to the patient’s deficient cells[10]. Unfortunately,
this therapy resulted efficient only in MPS I patients younger than nine months of
age, and presents important issues related to donor selection and adverse effects of
transplantation[11, 12]. HSCT is currently the standard care for MPS I younger
patients, despite the high risks of mortality, and it has been also used for other LSDs
without available treatments, such as KD[1].
Recently, gene therapy methods, such as gene replacement, antisense oligonucleotide (ASO) therapies, or gene editing, are also emerging for the treatment of
LSDs[13–15].
In particular, some variants of adenoassociated virus (AAV) resulted in excellent
vectors to deliver the gene encoding the deficient enzymes to cells[16]. Once manufactured for human use, the vector can be directly infused into the bloodstream or
target organs (direct gene transfer) or it can be applied ex‐vivo to stem cells in culture
and subsequently reimplanted into the donor (indirect gene transfer) [1]. These
cuttingedge therapies, all still in clinical trials for very few disorders, seem to be
effective in achieving remission of neurological manifestations. However, the major
drawbacks are related to establishing safe administration regime, the invasive route
of administration, and the difficulty of foreseeing longterm undesired effects[17].
Enzyme replacement therapy (ERT) consists of intravenous infusion of a recombinant wildtype enzyme, which enters the cell through membrane receptors (typically the mannose6phosphate receptor (M6PR)) and replaces the catalytic action
of the mutated lysosomal enzyme[18, 19]. Available since 1996[20], ERT is the only
treatment approved for many LSDs (Table9.1). The main limitations associated
with ERT are its high cost, possible immunologic response, and frequent hospitalization of patients, which have been now partially overcome with domiciliary treatments. In addition, the recombinant enzymes used are not able to cross the
blood–brain barrier (BBB) and reach the CNS, so that ERT lacks efficacy in neuronopathic forms of disorders.
The aim of the substrate reduction therapy (SRT) is to reduce the storage in the
lysosomes of undegraded substrates by using small molecules able to inhibit their
9.2 Available Treatments for LSDs: The Role of Carbohydrate-Based Therapeutics
macological
biosynthesis[21]. SRT could be in principle developed for all LSDs, but, to date, is
approved for Gaucher and NP diseases only, because, for these glycosphingolipids,
the knowledge of their biosynthetic pathways allowed to identify specific inhibitors
of the enzyme that synthesizes the substrates or one of their precursors.
Although trials in other glycosphingolipids and MPS are ongoing, the limitation
to very few pathologies represents the main drawback of SRT. In addition, SRT has
a slower efficacy than ERT and several side effects or complications, such as diarrhea, flatulence, abdominal pain, or weight loss.
In several LSDs, gene mutations result in lysosomal enzymes prone to a rapid
turnover or with a reduced catalytic activity [22]. However, since the substrate
accumulation occurs when residual enzyme activity decays below a certain threshold, a small improvement in the enzyme activity can be sufficient to slow down the
disease progression and to ameliorate clinical evidence. This is the concept on
which the pharmacological chaperone therapy (PCT) is based: it employs small
molecules (pharmacological chaperones, PCs) able to stabilize the tertiary structure of the mutant enzyme, thus improving its catalytic activity and reducing the
storage in the lysosomes (Figure9.2).
The main advantages of PCs are their lowcost, the oral administration, and the
possibility to cross the BBB exerting their action on the CNS[23].
Several PCs were designed and synthesized based on the structure of the natural
substrate or alternatively identified by highthroughput screening (HTS) studies. To
date, the only PC that reached the drug market is Migalastat (Galafold®, Amicus
249
Lysosome
Golgi
Stabilized
protein
ER
Figure9.2 Cartoon representing the Pharmacological Chaperone (PC) concept. Source:
Reproduced with permission from Boyd etal.[23]. © 2013 American Chemical Society
Publications.
PC
Misfolded
protein
Phar
chaperone (PC)
ERAD
Autophagy
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250
Therapeutics)[24, 25] for the treatment of Fabry disease (FD), but many other
compounds showed encouraging results in preclinical studies for the treatment of
other LSDs, including GD, Pompe disease, or gangliosidosis.
Surprisingly, except for cellbased therapies (both the oldest bone marrow transplantation and the emerging gene therapy), the most currently employed treatments, namely ERT, SRT, and PCT, all involve the use of iminosugars, sugar analogs
with a nitrogen replacing the endocyclic oxygen atom[26].
Iminosugars have been isolated from natural sources, including plants and microorganisms, for over 50 years. A great effort for their total synthesis has been pursued
by many researchers all around the world. For their synthesis, which is very challenging due to the presence of several contiguous stereocenters with a welldefined
configuration, both chiral pool strategies and enantioselective syntheses have been
developed[27–33].
Their resemblance to carbohydrates allows them to be recognized by, and interact
with carbohydrate receptors. In particular, iminosugars have been extensively investigated in the last 30
years as glycosidase [34–36] and glycosyltransferase
inhibitors[37–41].
They are more chemically and metabolically stable than their parent sugars, are
highly water soluble, can cross the BBB, and are normally excreted from the body
unmodified. Such properties distinguish them from other small polar molecules
and provide significant advantages for their use as drugs, especially in the treatment
of LSDs.
9.2.1 Enzyme Replacement Therapy (ERT)
In the 1980s the ingestion of plants belonging to the Leguminosae family was considered responsible for inducing lysosomal storage diseases in animals (e.g. swainsona toxicosis [42] or locoweed poisoning[43]) because of their high content of
iminosugars able to inhibit lysosomal glycosidases.
The observation that the endogenous enzyme activity was restored when the
inhibitor was removed[44] suggested that the lysosomal storage caused by the deficient enzyme in human LSD patients could be faced by supplying a natural or
recombinant functional enzyme, thus paving the way for the introduction of
the ERT.
ERT is based on the replacement of the mutated protein with normal protein to
restore its wildtype function. Although initially purified from human tissues, nowadays the deficient enzymes are produced by recombinant DNA technology.
Mammalderived cells, such as Chinese Hamster Ovary (CHO) cells, are the most
widely used platform to produce ERT recombinant enzymes[45].
After their efficacy and safety have been confirmed by clinical trials, to date, there
are 10 approved therapeutic enzymes as ERT for 7LSDs (Table9.1). Nevertheless,
there are some drawbacks related to this therapy, such as immune reactions, low
efficiency of lysosomal targeting, and difficulty reaching CNS. To overcome these
limitations, several strategies have been developed with the aim of increasing the
targeting efficiency and the ability to cross the BBB. Many of these approaches
9.2 Available Treatments for LSDs: The Role of Carbohydrate-Based Therapeutics
HO
HO
HO
OH
Migalastat, Galafold
OH
-butyl-deoxygalactonojirimycin
251
O
N
N
H
H
HO
Kifunensine (1)
O
HO
OH
HO
Castanospermine (2)
H
N
HO
OH
HO OH
N
Deoxygalactonojirimycin
H
(DGJ) (5)
HO
N
N-butyl-deoxynojirimycin
(N-butyl-DNJ) (3)
Miglustat, Zavesca
HO
OH
TM
OH
OH
TM
OH
N
H
Isofagomine
(IFG) (6)
N
OH
HO
(N-butyl-DGJ) (4)
OH
N
Lucerastat
OH
Figure9.3 Structures of iminosugars involved in ERT, SRT, and PCT for LSDs.
involve glycan remodeling in order to improve the cellular uptake and lysosomal
targeting.
Indeed, recombinant human lysosomal enzymes for ERT contain a mixture of
Nlinked glycans, including complex, hybrid, and at least one high mannose with
the mannose6phosphate (M6P) motif, which allow the enzyme to enter the cell by
exploiting the interaction between M6P and M6PR[46]. The only exception is ERT for
GD, which does not utilize the M6PR system but targets the enzyme to the mannose
receptor on macrophages. For this reason, recombinant βglucocerebrosidase has to
be processed chemically and with high costs after production, to expose mannose
residues on its Nlinked glycans. Today, this goal is achieved by treating the culture
®
medium of recombinant βglucocerebrosidase (Velaglucerase alpha, VPRIV
, Shire
Human Genetic therapies Inc.) with the iminosugar kifunensine (1, Figure 9.3),
obtaining high mannosetype glycans[47].
More recently, a further improvement in the ERT was provided by nanomedicine:
nanocarriers, such as liposomes or biocompatible polymers, are being developed
for sustained delivery and efficient intracellular targeting[48].
9.2.2 Substrate Reduction Therapy (SRT)
In the late 1980s Winchester and coworkers reported that coincubating wild
type fibroblasts with castanospermine (2, Figure9.3), a powerful inhibitor of αand
β glucosidases, resulted in the appearance of additional glycosphingolipids in the
treated cells, providing the first evidence that an iminosugar could affect the
metabolism of glycosphingolipids [49]. Few years later, Butter and coworkers
demonstrated that the αglucosidase (αglu) inhibitor Nbutyldeoxynojirimycin
(NbutylDNJ, 3, Figure 9.3) also inhibited ceramide glucosyltransferase, the
enzyme involved in the first step of the biosynthesis of most glycosphingolipids,
thus acting as a mimic of ceramide[50]. This suggested that N‐butylDNJ (3) could
be used as drug for reducing the synthesis of all glycosphingolipids for which glucosylceramide is the precursor and consequently decrease their accumulation in
those disorders where their degradation is impaired; this is the basis of SRT.
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252
NbutylDNJ (3), known under the commercial name of Zavesca® (Miglustat,
ActelionJanssenCilag International) has been licensed in Europe (2002) and USA
(2003) for treatment of nonneuronopathic GD type 1. Unfortunately, NbutylDNJ
(3) did not have any appreciable effect on the neurological manifestations of neuronopathic GD type[51]. Clinical trials of SRT with NbutylDNJ (3) for other glycosphingolipidoses, such as Fabry, Tay–Sachs, and Sandhoff diseases (SDs), did
not show clinical benefit and have been abandoned. Conversely, NbutylDNJ (3)
has been approved by EMA in 2009 for the treatment of Niemann–Pick type C
(NPC) disease[52, 53], for which it was proven to cross the BBB and slow the progression of neurological symptoms[54, 55]. Several other compounds including
some iminosugars are being evaluated in preclinical trials for Fabry (Nbutyl
deoxygalactonojirimycin, 4) disease and MPS III. Finally, SRT can be combined
with other therapies in view of a more personalized care program for patients[56].
9.2.3 Pharmacological Chaperone Therapy (PCT)
Although among enzymologists it was known that simple sugars protected glycosidases during their extraction from tissues, and it was observed that sometimes iminosugars activated rather than inhibited glycosidases invitro assays, suggesting
some stabilization[57]. It was only in 1999 that the concept of “active sitespecific
chaperone” (ASSC) was introduced[58].
An ASSC, later referred to as PC, is a small molecule able to bind the misfolded
enzyme in the endoplasmic reticulum (ER) and, by serving as a folding template,
prevent its premature degradation by endoplasmic reticulumassociated degradation (ERAD) pathway. The PC stabilization favors the trafficking of the properly
folded mutant enzyme to the Golgi apparatus for maturation, and the final transport
to the lysosome. Once the complex enzymePC reaches the lysosome, the PC is
replaced by the natural substrate (present in high concentration due to the pathology), which starts to be hydrolyzed, thus resulting in a rescue of the enzyme
activity[59–62].
Albeit it may seem contradictory, most PCs have also been competitive inhibitors of
their target enzyme[63] and were essentially identified among sugar mimetics[64].
Indeed, in their seminal work Fan and collaborators showed that subinhibitory
concentrations of the potent αgalactosidase inhibitor deoxygalactonojirimycin (DGJ,
5, Figure 9.3) increased the residual activity in lymphoblasts from FD patients, by
seven to eightfold over five days. Surprisingly, even infusions of simple
a weak inhibitor of the αgalactosidase, led to clinical improvement in a Fabry patient
with residual enzyme activity, supporting the use of chaperone mediated therapy for
lateonset LSDs[65]. DGJ (5) was considered a better candidate as a practicable therapeutic agent because of its higher affinity than galactose toward mutant lysosomal
αgalactosidase[66]. Nevertheless, almost 20 years of further studies were necessary
for DGJ (5) to pass Phase III clinical trial and be approved and commercialized as
TM
Galfold
(Migalastat, Amicus Therapeutics Inc.)[67] by EMA (2016)[68] and subse-
quently by FDA (2018)[69].
galactose,
9.2 Available Treatments for LSDs: The Role of Carbohydrate-Based Therapeutics
To date, Galafold is the only approved PC drug for the treatment of a LSD that is
FD. For GD, caused by mutated enzyme acidβglucosidase (glucocerebrosidase or
GCase), many efforts have been made to develop a PC, especially with the natural
iminosugar isofagomine (IFG (6), Figure9.3). Unfortunately, 6, although appearing
very promising in stabilizing mutant GCase[70], did not succeed in significantly
reducing the accumulation of glucosylceramide[71, 72].
In general, major difficulties in the translation to clinic are due to the identification of optimal dosing and regimen of PCbased drug administration, which aim to
maximize the enhancing effect (leading to substrate turnover) while avoiding the
inhibitory effect[22, 73, 74].
This goal can be achieved through different options, the first being an administration of the PC at alternative intervals (also called “onoff administration”), which
results in greater substrate clearance with respect to daily administration.
A more original and less investigated alternative is the development of pH
responsive PCs able to change their affinity for the mutated enzyme from the ER
(pH=7) to the lysosome (pH=5). For example, Ortiz Mellet and coworkers recently
reported that the insertion of an acidlabile moiety onto the iminosugar scaffold
allows the PC to switch from amphiphilic (higher affinity for the mutated enzyme)
to hydrophilic (low affinity for the mutated enzyme) in this biologically useful pH
window. Therefore, the PC stabilizes the enzyme in the ER, while it experiences
selfinactivation once it reaches the lysosome, not interfering with substrate
processing[75].
Alternatively, it is possible to use PCs directed to a lysosomal enzyme domain,
which does not include the active site but has a crucial role in the stabilization of the
protein (allosteric sites). In particular, in the last few years, the combination of
experimental multiple solvent crystal structure methods and computational fragment mapping allowed to identify previously unknown binding pockets for several
lysosomal enzymes. These studies led to the development of noncompetitive inhibitory PCs (which do not bind to the active site, offering an interesting alternative to
ASSC) or, even better, of noninhibitory PCs able to guarantee the refolding function, without affecting the enzyme hydrolytic action. While the former usually
belong to the family of iminosugars[76], the latter are not sugar mimetic compounds and are therefore out of the scope of this chapter[77]. However, if on one
hand, the noninhibitory PCs limit the risk of adverse enzymatic inhibition, their
identification is extremely challenging and can be pursued only through a massive
screeningbased approach.
Due to the several available options for the design and the use of PCs, as well as
the early stage understanding of their mechanism of action, it is not surprising that
most of the studies on iminosugars for the treatment of LSDs reported in this chapter focused on the development of PCs. In particular, the structure of the most
promising compounds, together with their development phases (invivo studies, in
vitro studies, clinical trials, approval)[78, 79] for selected MPS, sphingolipidoses,
glycogen storage disorders, and glycoproteinoses have been discussed.
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254
9.2.4 Combined ERT/PC Therapy
For those LSDs for which the ERT is available, low halflife and uneven distribution
of the recombinant enzyme remain inherent disadvantages of this symptomatic
treatment and are responsible for frequent patients’ hospitalization and high governmental costs. A promising solution that is being explored is the coadministration
of ERT with PCs, which is able to increase the lifetime of the infused enzyme, resulting in improved rate of substrate degradation and consequently ameliorating the
treatment efficacy. For example, significant increase in αgalactosidase activity in
the blood and the skin has been observed in Fabry patients orally treated with
Galafold (DGJ, 5) before the ERT infusion[21, 23].
9.3 Mucopolysaccharidoses
The MPS are genetic disorders caused by the deficiency of 1 of 11lysosomal enzymes
involved in the metabolism of glycosaminoglycans (GAGs) (Table 9.1). Enzyme
deficiencies may lead to the accumulation of the GAGs heparan sulfate (HS), dermatan sulfate (DS), keratan sulfate (KS), chondroitin sulfate (CS), or hyaluronan
within the lysosomes. The clinical phenotype of MPS varies and is characterized by
progressive multisystemic involvement affecting the brain, eye, ear, upper and lower
airways, liver, spleen, heart, bone, cartilage, and joints[80]. Currently, intravenous
ERT is the standard of care for nonneurological manifestations of MPS I, MPS II,
MPS IVA, MPS VI, and MPS VII patients (Table9.2). ERTs have been shown to be
Table9.2 Classification ofthe mucopolysaccharidoses (MPS) and available ERT.
Disorder Enzyme deficiency ERT
MPS I (Hurler,
Hurler–Scheie, and
Scheie syndrome;
Xlinked disease)
MPS II (Hunter
syndrome)
MPS IVA (Morquio A
syndrome)
MPS VI (Maroteaux–
Lamy syndrome)
MPS VII (Sly syndrome) βglucuronidase Vestronidase alfa (Mepsevii™;
αliduronidase Laronidase (Aldurazyme
Europe B.V., Gooimeer 10, NL1411
DD Naarden, The Netherlands),
available since 2003
Iduronate2sulfatase
(IDS)
Nacetyl
glucosamine6
sulfatase sulfatase
(GALNS)
Arylsulfatase B Galsulphase (Naglazyme
Recombinant human idursulphase
(Elaprase
Therapies, Inc., Cambridge, MA, USA),
available since 2006
Elosulphase alpha (Vimizim™;
BioMarin Pharmaceutical, Inc.,
Novato, CA, USA), available since 2014
Pharmaceutical, Inc., Novato, CA,
USA), available since 2005
Ultragenyx Pharmaceutical Inc.,
Novato, CA, USA), approved in 2018
®
; Shire Human Genetic
®
®
; Genzyme
; BioMarin
9.3 Mucopolysaccharidoses
effective in reducing urinary GAGs and liver and spleen volume for attenuate and
severe phenotypes [81]. Although intravenous ERT is an effective treatment, the
amelioration of CNS with ERT is limited by the BBB. HSCT is primarily used to treat
CNS manifestations in MPS I Hurler[82], while this effect is less clear for the other
MPS disorders[83].
Promising new therapeutic strategies, including intrathecal/intracerebroventricular injection of recombinant enzyme, the gene therapy, and nanotechnology as
enzyme and nucleic acid delivery systems have been developed and are being tested.
In vivo tests and clinical trials are in progress to determine the safety and efficacy of
these strategies in MPS patients[84–86].
Recently, a PC was described as a potential treatment alternative for MPS.
In the case of MPS II, a sulfated disaccharide derived from heparin 7 (D2S0,
Δ unsaturated 2sulfouronic acidNsulfoglucosamine), which mimics the struc-
ture of the natural substrate of IDS, was described as a potential PC for this enzyme
(Table 9.3). The compound was a competitive inhibitor (IC
of 30.1 μM) and
50
increased the thermal stability of human IDS invitro and improved the function of
intracellular IDS between 1.6 and 39.6fold in a mutationdependent manner in
patient fibroblasts and HEK293T cells expressing mutated IDS without a significant reduction in GAG levels[87]. MPS III is characterized by four types: MPS IIIA,
IIIB, IIIC, and IIID. For MPS IIIB, α (8), and β (9) Nacetylglucosaminidases
(NAGLU), a weak competitive inhibitor of NAGLU, demonstrated a significant PC
activity on mutant NAGLU (Table9.3)[88]. For MPS IIIC, the use of glucosamine
10, a competitive inhibitor of the enzyme, allowed for an increase of between
1.2 and 2.5fold in the activity of glucosaminide Nacetyltransferase in eight out of
nine tested mutations (Table9.3)[89]. For MPS IVA, three GALNS PCs, bromocriptine, ezetimibe, and pranlukast (noncarbohydratebased compounds), were identified by molecular dockingbased virtual screening[90, 91].
The identification of inhibitors of MPS recombinant enzymes represents a starting point for the development of enzyme stabilizers not only for PCT but also for
combined PC/ER therapy. In this context, Cardona and coworkers investigated the
multivalent effect. This phenomenon is defined as the increase in the biological
response obtained with compounds that possess more than one bioactive unit linked
to a common scaffold, compared to the sum of the contributions given by the individual bioactive molecules[92]. The multivalent effect can be quantified by determining the affinity enhancement per bioactive unit (rp/n), namely by dividing the
relative potency (rp) by valency (n). Dimers and trimers of DAB1 (1,4 dideoxy1,4
iminoarabinitol) (compounds 11 and 12)[93] and other multimeric iminosugars
such as the nonavalent compounds 13 and 14 [94], and gold glyconanoparticles
(AuGNPs) based on the same bioactive unit (15) were identified as excellent selective inhibitors of GALNS[95], together with simple AuGNPs decorated with monosaccharides and/or sulfateended ligands (16, unpublished data). The best results
are reported in the Table9.4.
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