Future
Editorial
For reprint orders, please contact reprints@future-science.com
Medicinal
Chemistry
Metallohelices: potential mimetics of
α-helical peptides in cancer treatment?
“
The design of nonpeptide systems that are able to imitate some of the functions
and structures of α-helices, but with more desirable drug-like properties is an area of
intense investigation.
”
Keywords: amphipathic • helicate • non-peptide mimetic • peptidomimetic • phenotypic Many traditional anticancer agents act by
damaging cellular DNA, but the lack of
selectivity offered by this approach commonly leads to undesirable side effects [1] .
In addition, the genetic instability of cancer
cells facilitates the development of resistance
to particular drug pathways [2] . There is thus
a drive to discover systems that exhibit different – perhaps more subtle – modes of action.
Host defense peptides, which are expected to
act by prohibiting cellular processes through
reversible and selective biomolecular interactions, have been investigated as potential
anticancer agents to this aim [3] . However,
these innate proteins are not ideal drug candidates as, although their pharmacodynamic
profiles are promising, they tend to suffer
from poor pharmacokinetics [4] . The design
of nonpeptide systems that are able to imitate some of the functions and structures of
α-helices, but with more desirable drug-like
properties is an area of intense investigation [5] . Such peptidomimetics could nevertheless be as costly and difficult to synthesize
as peptides [6] , and they commonly have low
solubility in aqueous media [7] . In addition,
the exquisite 3D architecture of the α-helix
is difficult to achieve realistically using the
rather flat topographies of conventional
synthetic organic chemistry.
Helicates: a problem of
stereochemistry, functionality
& stability
Helicates – assemblies of rigid organic
ligands around a metal ion core – resemble
α-helices in terms of their diameter and
charge [8] . The idea that metallohelices may
10.4155/FMC.14.150 © 2015 Future Science Ltd
function as peptidomimetics – including
synthetic and biophysical studies of their
potential in the biological regime – has been
advanced by groups worldwide [9] . Until
recently, however, these studies have been
limited to rather few compounds. Hannon
and colleagues reported a DNA-binding
iron(II) helicate with antimicrobial [10] and
anticancer [11] activity. An arginine-decorated
analog showed slightly higher cytotoxicity
than the parent compound against A2780
ovarian cancer cells (IC50 : ∼7 μM) [12] . The
parent helicate is of a comparable size to a key
α-helical motif of the amyloid-β peptide, a
therapeutic target in Alzheimer’s disease [13] .
Qu has discovered that this indeed strongly
binds to the protein and inhibits its aggregation [13] . The interaction appears to reduce
the cytotoxicity of amyloid-β in PC12 rat
pheochromocytoma cells; treatment with
the helicate alone did not affect cell viability.
In vivo studies showed that the compound
ameliorated memory deficits in a transgenic
mouse model [13] .
Certainly helicates have provided some
preliminary results in these early phenotypic
studies, but can they become realistic drug
candidates? They are not like anything in the
clinic, which is at once exciting and worrying.
They are ‘small’ molecules rather than ‘biologicals’, but actually are assembled from a collection of small-molecule subunits, and the application of Lipinski’s rule of five would seem
rather naive. Nevertheless, in order to move
towards serious preclinical studies, a number
of practical criteria must be achieved. These
criteria are common to all synthetic drugs, but
in our view, not enough attention has been
Future Med. Chem. (2015) 7(1), 1–4
Rebecca A Kaner
Institute of Advanced Study, University of
Warwick, Coventry, CV4 7HS, UK
and
Department of Chemistry, University of
Warwick, Coventry, CV4 7AL, UK
Peter Scott
Author for correspondence:
Department of Chemistry, University of
Warwick, Coventry, CV4 7AL, UK
peter.scott@warwick.ac.uk
part of
ISSN 1756-8919
1
Editorial Kaner & Scott
paid to this in helicate chemistry. The compounds must
be: optically pure and nonracemizing for use in human
systems; soluble and stable in aqueous solutions; readily
available on a practical scale; and synthetically flexible
so that properties can be optimized [14] .
“
It is nearly 30 years since Lehn et al. posited the
idea that helicates bore a resemblance to natural
α-helices, but it is only recently that they have
been approaching the point at which they may be
considered to be α-helix mimetic.
”
These are rather stringent targets for coordination
chemistry, where, for example, the art of enantioselective synthesis lags far behind that of carbon chemistry
and functionalized complexes are rather rare. In order
to make structurally robust complexes, it is conventional to use inert metal ions, such as ruthenium(II).
Aside from the inherent toxicity, this causes a synthetic
problem for helicates, in that kinetic mixtures of complexes are always formed. Laborious separations are
then necessary, leading to poor chemical yields [15] .
Furthermore, in order to reduce the number of possible isomers formed, helicate self-assembly uses highly
symmetrical building blocks. Hence, the idea of the
deliberate placement of α-helix mimetic recognition
elements might seem rather fanciful. Nevertheless,
recent reports have moved us significantly closer to
realizing the above targets.
Optically pure, functionalized & more stable
metallohelices
We first addressed the problems of optical purity, synthetic flexibility and stability in water. Our strategy for
metallohelix self-assembly was inspired by Lehn et al.
[8] , but does not rely on the use of rigid ligands. The key
was the development of simple self-assembling, optically pure monometallic complexes utilizing amino
acid derivatives as the source of chirality [16] . These
complexes were connected together with linear linkers
to form helical bimetallic species known as ‘flexicates’
[17] . The compounds are readily water soluble and
remarkably stable in a variety of media.
Promising antimicrobial activity of certain flexicates
was observed towards methicillin-resistant Staphylococcus aureus and Escherichia coli, alongside low toxicity
to the nonmammalian model organism Caenorhabditis elegans [17] . Studies on protein mimetic–protein
interactions by Qu and colleagues indicate that one
flexicate enantioselectively inhibits amyloid-β through
targeting α/β-discordant stretches at the early steps
of aggregation, with this being the first time such an
observation has been made [18] . This is demonstrated
using fluorescent living cell-based screening and
multiple biophysical and biochemical approaches.
2
Future Med. Chem. (2015) 7(1)
DNA interaction studies showed the selectivity of
some compounds for 5´-CACATA and 5´-CACTAT
segments, as well as for stabilizing different three- and
four-way oligonucleotide junctions [19] . Enantiomer
effects were also noted, and ΛFe enantiomers were
found to be the stronger DNA binders. Other flexicates show no affinity for DNA, but are found to be
active in ovarian cancer (IC50 : ∼3.5 μM in A2780 ovarian cancer cells) [19] . Flexicates do not damage DNA
in cancerous cells like many traditional agents, but
act by inducing cell cycle arrest and apoptosis. They
show high activity in the habitually resistant cell line
A2780cis (cisplatin-resistant ovarian carcinoma) [19] .
While flexicates have very favorable properties and
display structure-dependent activity in various phenotypic screens, they are rather symmetrical and do not
provide the kind of topographies that can realistically
be described as peptidomimetic. As Albrecht has noted,
“…the development of a synthetic route to asymmetric
helicates … would create significantly more freedom
for tailoring the structure – which, for instance, could
enable the optimization of biological properties” [20] .
We have recently disclosed that low-symmetry systems,
which we call ‘triplex metallohelices’, are available
[21] . The desirable properties of the flexicate series are
retained, or indeed amplified, and the triplex architectures were found to be chemically and enantiomerically
very stable in water and various biological media. Crucially, the topographies presented are amphipathic, in
that they feature a hydrophobic face opposite a region
that may be functionalized with a range of polar recognition elements. The structures are thus far more
reminiscent of short peptidic α-helices [3] . Examples of
these triplex metallohelices were highly active against
colon cancer cell lines, but with lower activity against
others, and showed essentially no activity against bacteria. This kind of selectivity is very unusual and indicates a subtle biomimetic mechanism. Dramatic effects
on the cell cycle and the promotion of early apoptosis
were observed, but no DNA damage was detected in a
range of studies [21] .
Vázquez has recently reported an approach to asymmetric helicate synthesis using a single optically pure
ligand from solid-phase synthesis containing two hairpin turns that – according to calculations – predisposes
the formation of a bimetallic iron(II) helix [23] . The
complex is assembled in situ on a small scale and is
not isolated, but is detected by mass spectrometry. The
isomeric purity was investigated by circular dichroism,
but this merely shows that there is an enantiomeric
excess of one helicity [22] . In this preliminary communication, it was suggested that the helical complex may
bind to DNA, and there is evidence that it accessed the
intracellular environment of Vero cells [23] .
future science group
Metallohelices: potential mimetics of α-helical The chemistry underpinning these new approaches
will allow for much greater diversity in functionality
and structure than has been possible hitherto in the
general area of helicates, but there is a need for still
greater structural and synthetic diversity. The ability to
place functional groups at positions of choice in helicates to the degree that nature manages with peptidic
systems is still a few steps away.
Translation to the clinic?
It should be noted that the metallohelices described
here exist exclusively in their helical state (i.e., there is
not a detectable unfolded architecture until irreversible
hydrolysis eventually occurs). While this is advantageous in terms of equilibrium binding to α-helix recognition sites, we must be realistic regarding stability
in aqueous systems. With the exception of a few systems, there is currently little information on this in the
literature [17,21] .
A significant amount of preliminary work on biological activity is available, but this must be built upon
with more detailed mechanistic studies towards translation to the clinic. Quite appropriately, the work so
far has been based on phenotypic screening, asking the
simple question: do these metallohelices behave like
short antimicrobial/anticancer peptides? We feel that
the early focus in this area on DNA binding was perhaps rather opportunistic – such phenomena are rather
easy to study in a cell-free environment, and there is
little if any evidence that the compounds pass through
References
1
Fu D, Calvo JA, Samson LD. Balancing repair and tolerance
of DNA damage caused by alkylating agents. Nat. Rev.
Cancer 12(2), 104–120 (2012).
2
Baguley B. Multiple drug resistance mechanisms in cancer.
Mol. Biotechnol. 46(3), 308–316 (2010).
3
Gaspar D, Veiga AS, Castanho MA. From antimicrobial to
anticancer peptides. A review. Front. Microbiol. 4, 294 (2013).
4
Hancock REW, Sahl H-G. Antimicrobial and host-defense
peptides as new anti-infective therapeutic strategies. Nat.
Biotechnol. 24(12), 1551–1557 (2006).
the cell membrane. The potentially more fruitful field
of protein interactions needs far greater attention. A
wider exploration of disease areas is also warranted,
and the work of Qu and colleagues provides excellent examples [13,18] . Furthermore, few selectivity (as
opposed to mere activity) data have been presented for
anticancer metallohelices, either in terms of differential
activity between cancer cell lines [19] or organisms [21] .
It is nearly 30 years since Lehn et al. posited the idea
that helicates bore a resemblance to natural α-helices
[8] , but it is only recently that they have been approaching the point at which they may be considered to be
α-helix mimetic. The chirality, stability and functionality of the latest systems, alongside their potency and
selectivity against cancer cells, encourages us to be
hopeful that this will be achieved, and that they might
be translated to a clinical situation in this or other disease areas in which natural host defense peptides are
known to play a role.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial
interest in or financial conflict with the subject matter or
materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options,
expert testimony, grants or patents received or pending, or
royalties.
No writing assistance was utilized in the production of this
manuscript.
9
Maayan G, Albrecht M. Metallofoldamers: Supramolecular
Architectures from Helicates to Biomimetics. John Wiley &
Sons, Ltd, UK (2013).
10
Richards AD, Rodger A, Hannon MJ, Bolhuis A.
Antimicrobial activity of an iron triple helicate. Int. J.
Antimicrob. Agents 33(5), 469–472 (2009).
11
Hotze ACG, Hodges NJ, Hayden RE et al. Supramolecular
iron cylinder with unprecedented DNA binding is a
potent cytostatic and apoptotic agent without exhibiting
genotoxicity. Chem. Biol. 15(12), 1258–1267 (2008).
12
Cardo L, Sadovnikova V, Phongtongpasuk S, Hodges
NJ, Hannon MJ. Arginine conjugates of metallosupramolecular cylinders prescribe helicity and enhance
DNA junction binding and cellular activity. Chem.
Commun. 47(23), 6575–6577 (2011).
5
Wade J, Otvos L. Current challenges in peptide-based drug
discovery. Front. Chem. 2, 62 (2014).
6
Pattabiraman VR, Bode JW. Rethinking amide bond
synthesis. Nature 480(7378), 471–479 (2011).
13
7
Davis JM, Tsou LK, Hamilton AD. Synthetic non-peptide
mimetics of alpha-helices. Chem. Soc. Rev. 36(2), 326–334
(2007).
Yu H, Li M, Liu G et al. Metallosupramolecular complex
targeting an α/β discordant stretch of amyloid β peptide.
Chem. Sci. 3(11), 3145–3153 (2012).
14
8
Lehn JM, Rigault A, Siegel J, Harrowfield J, Chevrier B,
Moras D. Spontaneous assembly of double-stranded helicates
from oligobipyridine ligands and copper(I) cations: structure
of an inorganic double helix. Proc. Natl Acad. Sci. USA
84(9), 2565–2569 (1987).
Howson SE, Scott P. Approaches to the synthesis of
optically pure helicates. Dalton Trans. 40(40), 10268–
10277 (2011).
15
Pascu GI, Hotze ACG, Sanchez-Cano C, Kariuki BM,
Hannon MJ. Dinuclear ruthenium(II) triple-stranded
helicates: luminescent supramolecular cylinders that bind
future science group
Editorial
www.future-science.com
3
Editorial Kaner & Scott
the mechanism involve DNA binding? Chem. Sci. 4(12),
4407–4416 (2013).
and coil DNA and exhibit activity against cancer cell lines.
Angew. Chem. Int. Ed. 46(23), 4374–4378 (2007).
4
16
Howson SE, Allan LEN, Chmel NP, Clarkson GJ, Van
Gorkum R, Scott P. Self-assembling optically pure Fe(A–B) 3
chelates. Chem. Commun. 13, 1727–1729 (2009).
17
Howson SE, Bolhuis A, Brabec V et al. Optically pure, waterstable metallo-helical ‘flexicate’ assemblies with antibiotic
activity. Nat. Chem. 4(1), 31–36 (2012).
18
Li M, Howson SE, Dong K et al. Chiral metallo-helical
complexes enantioselectively target amyloid β for treating
Alzheimer’s disease. J. Am. Chem. Soc. 136(33), 11655–11663
(2014).
19
Brabec V, Howson SE, Kaner RA et al. Metallohelices
with activity against cisplatin-resistant cancer cells; does
Future Med. Chem. (2015) 7(1)
20
Albrecht M. Helicates: making head–head–tails of it. Nat.
Chem. 6(9), 761–762 (2014).
21
Faulkner AD, Kaner RA, Abdallah QMA et al. Asymmetric
triplex metallohelices with high and selective activity against
cancer cells. Nat. Chem. 6(9), 797–803 (2014).
22
Howson SE, Scott P. Comments on ‘Synthesis and
characterisation of enantiopure copper(II) complexes with
chiral bidentate ligands’. Dalton Trans. 40(16), 4332–4333
(2011).
23
Gamba I, Rama G, Ortega-Carrasco E et al. Programmed
stereoselective assembly of DNA-binding helical
metallopeptides. Chem. Commun. 50, 11097–11100 (2014).
future science group