Noteworthy Chemistry

October 6, 2008

Tune protein release profiles through architectural design
Deliver anticancer drugs using nanoscale coordination polymers
Here’s a hybrid approach to palladium removal
Synthesize resveratrol via Heck–Matsuda coupling
Dendritic pendants give polyacetylenes stability and functionality
Convert benzoquinones to useful functionalized cyclopentenones
Shape-memory behavior is POSS-itively promising

Tune protein release profiles through architectural design. P. Pitukmanorom, T.-H. Yong, and J. Y. Ying* at the Institute of Bioengineering and Nanotechnology (Singapore) detail a design strategy that controls material properties and processing parameters to influence the protein release characteristics of inorganic–polymer nanocomposite microspheres. They controlled the dissolution of the inorganic framework using the acidic breakdown products of the hydrolyzed encapsulating polymers.

The researchers obtained sustained release profiles by creating poly(D,L-lactic acid-co-glycolic acid) (PLGA) microspheres that surround calcium phosphate nanoparticles that have proteins adsorbed onto them. Varying PLGA molecular weight (MW) modulates the extent of linear, sustained release: Higher-MW PLGA (84 kDa) releases over an extended time period (10 weeks) compared with a 3 week release for lower-MW PLGA (64 kDa). Changing the lactic acid/glycolic acid ratio also modifies the degradation characteristics of the PLGA—and consequently the protein release behavior—because it alters the balance of hydrophobicity and hydrophilicity in the PLGA is altered. For example, higher hydrophobic lactic acid content leads to slower hydrolysis.

The authors also compared the release of bone morphogenetic protein 2 (BMP-2) from the inorganic–polymer nanocomposite microspheres with that achieved by existing technology (collagen sponges). PLGA (84 kDa)–calcium phosphate microspheres infused with BMP-2 exhibit sustain release of the protein over 7 weeks in contrast to the large initial burst and 2 week release period from the sponges; these microspheres also enhance protein bioactivity upon exposure to osteocalcin. By introducing a second polymer [poly(L-lactic acid), PLLA] that functions as a diffusion barrier to protein release, the researchers tailored the release characteristics to create a delayed release mechanism. The team produced PLLA core–PLGA shell microspheres by exploiting the phase separation phenomena of a mixture of PLLA and PLGA during solvent evaporation. Robust double-wall microspheres form with slow CH₂Cl₂ solvent removal.

The researchers note that PLGA MW and composition, which influence degradation rate and dictate the porosity of the PLLA shell, are key parameters in tuning the delayed protein release characteristics. For example, changing the MW from 7 to 24 kDa results in an increase in lag time from 2 to 6 weeks. This flexible material platform offers a unique opportunity to address current challenges in tissue engineering. (Adv. Mater. 2008, 20, 3504–3509; LaShanda Korley)

Deliver anticancer drugs using nanoscale coordination polymers. Researchers base the construction of nanoscale coordination polymers (NCPs) on a combination of metal ion connectors and polydentate ligands; the choice of building blocks is nearly limitless. If the careful choice of connectors and bridging ligands can optimize an NCP’s stability, then its inherent solubility in aqueous environments can allow the controlled release of biologically functional species. In fact, the U.S. Food and Drug Administration has approved several nanoparticle-based anticancer drugs, thus validating this nanotherapeutic approach.

W. Lin and co-workers at the University of North Carolina (Chapel Hill) observe that physicians still use platinum-based drugs in the primary treatment of several cancers. The researchers adapted the nanoparticle concept to deliver platinum-based drugs to cancer cells by including the drugs into NCPs. The strategy stabilized the platinum-based NCPs with shells of amorphous silica to prevent rapid dissolution, effectively controlling the release rate of the active platinum species.

The authors constructed NCPs from Tb³⁺ ions and disuccinatocisplatin bridging ligands (1), adjusted the pH of the resulting aqueous solution to ~5.5, and quickly added MeOH. The resulting NCPs precipitated from aqueous solution, and the authors isolated them by centrifuge, washed them with alcohol, and redispersed them in EtOH via ultrasonication. The diameter of these initial NCPs in EtOH was ~58 nm.

Treating the poly(vinylpyrrolidone)-functionalized NCP intermediates with Si(OEt)₄ in an aqueous mixture of NH₃ and EtOH stabilized them to prevent rapid dissolution in water. The resulting silica shell thickness typically varied from ~3 to ~7 nm. The coated NCPs had a diameter of ~53 nm.

The researchers assessed the controlled release behavior of these nanoparticles by dialyzing particle samples against a HEPES buffer (pH 7.4), and they efficiently controlled the release of the platinum species by varying the silica shell thickness. Typical dissolution half-lives were ~5.5–9 h, as compared with ~1 h for the unmodified NCPs. This work has important medical implications because the release rates of the modified nanoparticles allow sufficient time for the platinum-based NCPs to circulate within the body and accumulate in tumor tissue.

Subsequent in vitro cytotoxicity studies on the human colon carcinoma cell line HT-29 did not lead to appreciable cell death because no reductants were present in the media to convert species 1 to the active Pt(II) species. However, the authors note that the presence of reducing biomolecules such as glutathione would promote activity in vivo by reducing the released species to Pt(II). The researchers enhanced the cellular uptake of the modified NCPs by grafting a small cyclic peptide onto the particle surface; they obtained IC₅₀ values of 9.7–11.9 μM, compared with 13.0 μM for the cisplatin standard. (IC₅₀ is the 50% inhibitory concentration.) This suggested that receptor-mediated endocytosis sufficiently internalized the targeted NCPs.

The generality of this approach should allow researchers to design a wide variety of NCPs to deliver medically important therapeutic or imaging agents. (J. Am. Chem. Soc. 2008, 130, 11584–11585; W. Jerry Patterson)

Here’s a hybrid approach to palladium removal. A. G. Livingston and coauthors at Imperial College (London) and the Technical University of Lisbon report that using a combination of nanofiltration and a suitable adsorbent to remove palladium residues is more efficient than either method alone. In addition, the hybrid approach requires significantly less adsorbent; it obtains an 8.5-fold greater palladium reduction than the equivalent process using 10 times less adsorbent.

The authors believe that the two-stage hybrid process works by removing bulky ligated palladium and palladium nanoparticles during the membrane stage of the separation. (Org. Process Res. Dev. 2008, 12, 589–595.; Will Watson)

Synthesize resveratrol via Heck–Matsuda coupling. Resveratrol (1) is a phytoalexin found in red wine. Its long-term consumption is associated with a decrease in the risk of coronary heart disease (the so-called French paradox). Various researchers have used Wittig and Horner–Emmons reactions, palladium-catalyzed Heck and Suzuki couplings, and ruthenium-catalyzed metatheses to synthesize this compound.

C. R. D. Correia and co-workers at the State University of Campinas (Brazil) studied the arylation of styrenes with arenediazonium salts in the presence of palladium catalysts (the Heck–Matsuda reaction) to synthesize resveratrol and analogues such as the anticancer drug candidate DMU-212 (2). Potential advantages of the Heck–Matsuda reaction are that it avoids the use of phosphines and is faster, less expensive, and “greener” than competitive methods.

To prepare DMU-212, the authors optimized the reaction between 3,4,5-trimethoxyphenyldiazonium tetrafluoroborate (3) and p-methoxystyrene (4) in terms of solvent, palladium catalyst, catalyst loading, and temperature. The researchers attained the best conditions by using PhCN as solvent and Pd₂(dba)₃ (1 mol%) as catalyst at room temperature to produce compound 2 in 1 h; dba is dibenzylideneacetone. The Heck–Matsuda reaction was highly regio- and stereoselective, furnishing only the E-isomer.

The authors synthesized resveratrol by coupling arenediazonium salt 5 and p-acetoxystyrene (6) in the presence of Pd(OAc)₂ under an atmosphere of CO as an in situ reductant for Pd(II). Subsequent hydroxyl deprotection gave the desired natural product in high yield. (Tetrahedron Lett. 2008, 49, 5668–5671; José C. Barros)

Dendritic pendants give polyacetylenes stability and functionality. Poly(phenylacetylene) (PPA) is a well known conjugated polymer, but its notorious instability has hindered its usefulness, J. G. Rudick and V. Percec* summarize the work carried out in Percec’s group at the University of Pennsylvania (Philadelphia) to make stable PPA derivatives and to generate PPA-based functional materials.

The researchers attached self-organizable dendritic pendants to the PPA skeleton. The steric effect of the bulky dendrons disfavors the formation of the cisoid conformation in the resultant cis-PPA derivatives and retards or eliminates polymer degradation via a 6π-electrocyclization mechanism at elevated temperatures, thus making the PPAs thermally stable.

The authors processed the stable PPAs into aligned fibers by extrusion. Upon heating, the PPAs underwent cisoid-to-transoid isomerization. The conformational isomerism translated to unidirectional motion in the self-assembled dendronized PPAs, enabling the preparation of nanomechanical actuators capable of lifting macroscopic objects such as an American dime. (Macromol. Chem. Phys. 2008, 209, 1759–1768; Ben Zhong Tang )

Convert benzoquinones to useful functionalized cyclopentenones. Masked o-benzoquinones (MOBs) are versatile reactants that undergo [4 + 2] cycloaddition reactions with many dieneophiles and have significant potential for use in the total synthesis of natural products. Now T.-C. Kao, G. J. Chuang, and C.-C. Liao* at the National Tsing Hua University (Hsinchu, Taiwan) report that treating MOBs (e.g., 1) by photo-oxygenation to undergo ring contraction yields densely functionalized 4-hydroxy-2-cyclopentanones—a process that depends critically on the choice of solvents.

Whereas CHCl₃ primarily promotes the formation of endoperoxides, the use of MeOH exclusively forms the desired cyclopentenones, typified by structure 2. The researchers prepared MOB reactants by oxidizing the corresponding 2-methoxyphenols with diacetoxyiodobenzene in MeOH.

The authors used rose bengal as a sensitizer. They carried out the reaction by bubbling oxygen into the MOB solution during irradiation. Upon completion, they treated the solution with thiourea to generate 2. They expanded their process by a final treatment with NaHCO₃, which modified the cyclopentenone scaffold to create a chiral epoxide and an orthoester group (3).

In an interesting application of this method, the authors used a variant of 2 to complete a short synthesis of (±)-untenone A (4), a natural product with inhibitory activity against cell proliferation of L1210 leukemia and mammalian DNA polymerases. Structure 4 is only one example of several biologically active products that contain the featured 4-hydroxy-2-cyclopentenone moieties. (Angew. Chem., Int. Ed. 2008, 47, 7325–7327; W. Jerry Patterson)

Shape-memory behavior is POSS-itively promising. P. T. Mather and coauthors at Case Western Reserve University (Cleveland, OH) and Syracuse University (NY) investigated the shape-memory behavior of polyurethane elastomers composed of a polyhedral oligosilsesquioxane (POSS)–modified hard block and a biodegradable poly(D,L-lactide)-containing soft domain. By copolymerizing the polylactide with 600–4000 molecular weight (MW) poly(ethylene glycol) (PEG), the authors determined that copolymer (P1k)LA (PEG with MW 1000 and polylactide with MW ~11 kDa) has the most promising combination of hydrophilicity and thermal behavior (T_g ≈ 29 °C) necessary for biomedical applications.

The authors obtained segmented polyurethanes with hard block contents varying from ~10 to 21 wt% via one-pot polymerization of (P1k)LA with a lysine-derived diisocyanate (LDI) and a POSS diol chain extender with isobutyl groups at the vertices. The team achieved enhanced microphase separation as the POSS-contained hard segment weight increased as a result of the increasing crystallinity of the POSS units within the hard domain. Based on detailed analysis of thermal behavior, microstructure, and processibility, the researchers focused on (P1k)LA-P3 (~15 wt% hard segment; ~87 kDa; T_g ≈ 34 °C; mp ≈ 118 °C) for examining thermally induced, one-shape memory behavior.

Films of (P1k)LA-P3 show almost complete shape fixing (>99%) and significant recovery (~71%) during the initial shape-memory cycle. Shape fixing and shape recovery improved under repeated cycling. Thinking ahead toward biomedical devices, the authors also investigated in vitro degradation characteristics; they show that hydrolytic cleavage of the ester linkage in the soft domain leads to bulk degradation after a lag time of ~30 days without significant water uptake. (Biomacromolecules 2008, 9, 2458–2467; LaShanda Korley)