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Page 2 of 35 Scherman. Rare Dis Orphan Drugs J 2023;2:12 https://dx.doi.org/10.20517/rdodj.2023.01

INTRODUCTION
In the last century, Paul Ehrlich and Emil Fischer introduced the concepts of chemotherapy, magic bullet,
and lock-key, in which a drug is defined as a small molecule that specifically binds to a biological target
through a three-dimension spatial recognition pattern . With the advancement of genetics and the
[1,2]
sequencing of the human genome, the genetic defects being the cause of rare genetic diseases are being
increasingly elucidated. This has led to the emergence of a new class of “genetic” drugs. In addition to
binding to their target through a 3-dimensional hydrogen bonding recognition pattern, these drugs
recognize a 1-dimensional linear genetic sequence. In such cases, drug design is based on genetic
information, which opens the perspective of the fast and rational development of drugs against a
considerable number of diseases characterized by a specific genetic defect.

Theoretically, any gene or mRNA can be targeted by a proper nucleotide sequence selected to be unique on
a given genome or transcriptome. Thus, such a sequence recognition mechanism opens tremendous
perspectives in medicinal chemistry. Instead of a painstaking customized search for specific spatial
recognition of a targeted biological ligand by a chemical compound or a monoclonal antibody, genetic
drugs are based on a robust universal platform that can be used for a very large number of applications.
Protein targets that were until now considered “non-druggable” can be challenged on their genetic
expression. In addition, the use of a common validated technology for all antisense oligonucleotides (ASO)
or all small interfering RNA (siRNA) allows for shortening pharmaceutical development steps and reducing
costs, which is of paramount importance for rare diseases. In the case of ultrarare diseases, personalized
therapy can now be envisioned with these revolutionary platforms for an n-of-1 patient .
[3]

The basic concepts, principles, mechanisms of action, and chemical optimizations of ASOs and siRNAs are
illustrated in the present review. These RNA-targeted drugs can cause RNA degradation or act as steric
blockers. In the latter case, they can inhibit RNA translation, antagonize a miRNA, modulate splicing, or
induce exon skipping. Their specific divergent functions correspond to adapted chemical and delivery
optimizations. Thus, RNA drugs are able either to restore a therapeutic mRNA or, in most cases, suppress
mRNA or block mRNA translation to correct “gain-of-function” dominant genetic disorders. But another
important application is that of splice modulator, which can overcome a nonsense mutation and lead to the
expression of a functional, although shortened, therapeutic protein.

Out of the scope of the present review are the cell and gene therapy approaches compensating for a genetic
deficiency by replacing the wild-type correct gene. We will not detail either the more recent revolution of
genome editing, in which a genetic defect is corrected “in situ” by tools such as clustered regularly
interspaced short palindromic repeats (CRISPR) associated with the Cas9 (CRISPR-Cas9) . Finally, the
[4]
present review does not intend at an extensive description of the clinical trials realized so far, as this aspect
has been extensively reviewed elsewhere .
[5-8]

REVIEW
Steric blocker ASOs
The first RNA drug was introduced in 1978 against Rous sarcoma viral replication [9,10] . It used a 13-mer
synthetic antisense oligonucleotide (ASO). It was an oligodeoxynucleotide complementary to 13 nucleotides
of the 3’- and 5’-reiterated terminal sequences of Rous sarcoma virus 35S RNA and annealing to it through
Watson-Crick recognition. This creates an intracellular DNA/RNA heteroduplex which may either function
as a translation steric blocker or lead to mRNA degradation. The ASOs acting as steric blockers of
translation target the mRNA translation initiation site (start codon). This inhibits the binding of ribosomal
subunits to the mRNA through steric hindrance, thus blocking protein synthesis [Figure 1A]. While this