Chemistry Reference
In-Depth Information
to release a molecular species (often a drug) via some cascade reactions from many
contiguous bond-breaking processes. By definition, a chemical adaptor or molecular
trigger is a molecular component inclusive of the dendrimer, that is especially
designed to initiate a directional cleavage of the latter at a starting point in the
macromolecule, after obeying to a stimuli (light, pH, metals, redox, etc.). In other
words, the resulting dendrimer is seen as a smart material or a molecular device ready
to fragment and to release a species when a specific signal is given. The specificity of
the signal, the efficiency in the sequential bond-breaking processes and the avoidance
of side reactions, are the key factors in those processes. It is important for some
medical applications such as in drug delivery, where the molecular fragments are well
defined and should be biocompatible, nontoxic, and easily degradable. Another
important point is their synthesis, which should avoid complicated strategies with
such sophisticated molecular systems.
Those concepts definitively launched the field of cleavable dendrimers and the
disassembly of dendritic backbones [190]. By analogy to polymers, it could be
compared to a depolymerization. Those chemical adaptors were incorporated in a
specific functional sequence and they can trigger a cascade dendritic cleavage in a
linear or in a geometric way, by using an external stimuli. This domino-type cleavage
could be promoted by a pH variation [90,115,135,139,148,191,192], photochemis-
try [58,108,193], transition metals [194,195], enzymes [54,134,141,142,196] (hy-
drolytic enzymes of esters, amides, and carbamates, etc.), catalytic antibo-
dies [184,191,197], a redox reaction [198], or a thermal process. Scheme 13.5 reports
a brief literature survey of some chemical adaptors, it was adapted from a previous
article [1].
The cleavage of dendrimers could proceed frompartial removal of a few functions,
removal of specific sequences leading to a functional macromolecule, removal of
dendrons (or a part of them), or some advanced cleavages of the backbone toward a
full (bio)degradation into simple chemical species. In most cases, the release of a
molecule for a specific application was sought (such as an anticancer drug delivery).
Alternatively, the remaining dendritic backbone could be of some utilities for further
applications.
SCHEME 13.5
Chemical adaptors units and the external stimuli involved in a cleavage. The
arrow represents the initiating part of the cascade reactions.
