(Z)-1,4-diamino-2-butene as a vector of boron, fluorine, or iodine for cancer therapy and imaging: synthesis and biological evaluation

Degradable polymers via olefin metathesis polymerization

The development of degradable polymers has commanded significant attention over the past half century. Approaches have predominantly relied on ring-opening polymerization of cyclic esters (e.g., lactones, lactides) and N-carboxyanhydrides, as well as radical ring-opening polymerizations of cyclic ketene acetals. In recent years, there has been a significant effort applied to expand the family of degradable polymers accessible via olefin metathesis polymerization. Given the excellent functional group tolerance of olefin metathesis polymerization reactions generally, a broad range of conceivable degradable moieties can be incorporated into appropriate monomers and thus into polymer backbones. This approach has proven particularly versatile in synthesizing a broad spectrum of degradable polymers including poly(ester), poly(amino acid), poly(acetal), poly(carbonate), poly(phosphoester), poly(phosphoramidate), poly(enol ether), poly(azobenzene), poly(disulfide), poly(sulfonate ester), poly(silyl ether), and poly(oxazinone) among others. In this review, we will highlight the main olefin metathesis polymerization strategies that have been used to access degradable polymers, including (i) acyclic diene metathesis polymerization, (ii) entropy-driven and (iii) enthalpy-driven ring-opening metathesis polymerization, as well as (iv) cascade enyne metathesis polymerization. In addition, the livingness or control of polymerization reactions via different strategies are highlighted and compared. Potential applications, challenges and future perspectives of this new library of degradable polyolefins are discussed. It is clear from recent and accelerating developments in this field that olefin metathesis polymerization represents a powerful synthetic tool towards degradable polymers with novel structures and properties inaccessible by other polymerization approaches.

Degradable Polyphosphoramidate via Ring-Opening Metathesis Polymerization

We report the synthesis of a degradable polyphosphoramidate via ring-opening metathesis polymerization (ROMP) with the Grubbs initiator (IMesH2)(C5H5N)2(Cl)2Ru═CHPh. Controlled ROMP of a low ring strain diazaphosphepine-based cyclic olefin was achieved at low temperatures to afford well-defined polymers that readily undergo degradation in acidic conditions via the cleavage of the acid-labile phosphoramidate linkages. The diazaphosphepine monomer was compatible in random and block copolymerizations with phenyl and oligo(ethylene glycol) bearing norbornenes. This approach introduced partial or complete degradability into the polymer backbones. With this chemistry, we accessed amphiphilic poly(diazaphosphepine-norbornene) copolymers that could be used to prepare micellar nanoparticles.

Probing the geometry reorganization from solution to gas-phase in putrescine derivatives by IRMPD, 1H-NMR and theoretical calculations

Geometry reorganization of ESI formed ions are demonstrated and explicit calculations of the solution phase are shown to be relevant.

Comparison of interactions of diamine and Mg²⁺ with RNA tertiary structures: similar versus differential effects on the stabilities of diverse RNA folds

Cations play a large role in stabilizing the native state of RNA in vivo. In addition to Mg²⁺, putrescine²⁺ is an abundant divalent cation in bacterial cells, but its effect on the folding of RNA tertiary structure has not been widely explored. In this study, we look at how the stabilities of four structured RNAs, each with a different degree of dependence on K⁺ and Mg²⁺, are affected by putrescine²⁺ relative to Mg²⁺. Through the use of thermal melts, we observe that (i) at a given concentration, putrescine²⁺ is less effective than Mg²⁺ at stabilizing RNA, (ii) the stability imparted to RNA by various diamines is a function of charge density (average separation distance between charges) as well as the flexibility of the counterion, and (iii) when Mg²⁺ is already present in a buffer, further addition of putrescine²⁺ may either destabilize or stabilize RNA structure, depending on whether the native RNA does or does not chelate Mg²⁺ ion, respectively. At ion concentrations likely to be found in vivo, the effect of putrescine²⁺ on the free energy of folding of an RNA tertiary structure is probably quite small compared to that of Mg²⁺, but the ability of mixed Mg²⁺/putrescine²⁺ environments to (in effect) discriminate between different RNA architectures suggests that, in some cells, the evolution of functional RNA structures may have been influenced by the presence of putrescine²⁺.

Structure-activity investigations of polyamine-anthracene conjugates and their uptake via the polyamine transporter

A series of polyamine conjugates were synthesized and evaluated for their ability to target the polyamine transporter (PAT) in two Chinese hamster ovary (CHO) cell lines (PAT-active CHO and PAT-inactive CHOMG). This systematic study identified salient features of the polyamine architecture required to target and enter cells via the PAT. Indeed, the separation of charges, the degree of N-alkylation, and the spacer unit connecting the N(1)-terminus to the appended cytotoxic component (anthracene) were found to be key contributors to optimal delivery via the PAT. Using the CHO screen, the homospermidine motif (e.g., 4,4-triamine) was identified as a polyamine vector, which could enable the selective import of large N(1)-substituents (i.e., naphthylmethyl, anthracenylmethyl and pyrenylmethyl), which were cytotoxic to cells. The cell selectivity of this approach was demonstrated in B-16 murine melanoma cells and normal melanocytes (Mel-A). Three polyamine areas (recognition and transport, vesicle sequestration and polyamine-target interactions) were identified for future research.

Synthesis of Hsp90 dimerization modulators

The synthesis and evaluation of several chemical modulators of heat shock protein 90 (Hsp90) dimerization is presented. These agents may represent useful tools to study the importance of N-terminal dimerization and also to determine subunit interface(s) in Hsp90.

Antizyme induction mediates feedback limitation of the incorporation of specific polyamine analogues in tissue culture

Spermidine, spermine and putrescine are essential for mammalian cell growth, and there has been a pervasive effort to synthesize analogues of these polyamines that will disrupt their function and serve as tools to inhibit cell proliferation. Recently, we demonstrated that a number of such polyamine analogues are also capable of inducing the regulatory protein AZ (antizyme). In the present study the incorporation of a few sample analogues [mimics of bis(ethyl)spermine] was shown to be significantly limited by a decrease in the V(max) for the polyamine transport system in response to analogue-induced AZ. This creates an unusual circumstance in which compounds that are being designed for therapeutic use actually inhibit their own incorporation into targeted cells. To explore the impact of this feedback system, cultures of rat hepatoma HTC cells were pre-treated to exhibit either low or high polyamine uptake activity and then exposed to polyamine analogues. As predicted, regardless of initial uptake activity, all cultures eventually achieved the same steady-state levels of the cellular analogue and AZ. Importantly, analogue-induced AZ levels remained elevated with respect to controls even after the native polyamines were reduced by more than 70%. To model the insufficient AZ expression found in certain tumours, GS-CHO (GS Chinese-hamster ovary) cells were transfected to express high levels of exogenic AZI (AZ inhibitor). As anticipated, this clone incorporated significantly higher levels of the polyamine analogues examined. This study reveals a potential limitation in the use of polyamine-based compounds as therapeutics, and strategies are presented to either circumvent or exploit this elegant transport feedback system.

Pharmacological aspects of cytotoxic polyamine analogs and derivatives for cancer therapy

During the past 20 years, numerous derivatives and analogues of spermidine (Spd) and spermine (Spm) were synthesized with the aim to generate a new type of anticancer drug. The common denominator of most cytotoxic polyamine analogues is their lipophilicity, which is superior to that of the parent amines. The natural polyamines bind to polyanions and to proteins with anionic binding sites. Their hydrophilicity/hydrophobicity is balanced, allowing them to perform physiological functions by interacting with some of these anionic structures, without impairing the functionality of others. Because the attachment of lipophilic substituents to the polyamine backbone increases the binding energy, lipophilic polyamine derivatives affect secondary and tertiary structures of a larger number of macromolecules than do their natural counterparts. In addition, lipophilicity improves the blood-brain barrier transport and thus enhances CNS toxicity. Close structural analogues of spermidine and spermine mimic the natural polyamines in regulatory functions. The cytotoxic mechanisms of analogues with a less close structural resemblance to spermidine or spermine have not been completely clarified. The displacement of spermidine from functional binding sites and the consequent prevention of its physiological roles is a likely mechanism, but many others may play a role as well. Up to now, polyamine analogues were conceived without specific growth-related targets in mind. To develop therapeutically useful drugs, it will be imperative to identify specific targets and to design compounds that interact selectively with the target molecules. It will also be necessary to include, at an early state of the work, pharmacological and toxicological considerations, to avoid unproductive directions.