d0 Tungsten alkylidyne alkyl complex (Me3SiCH2)3W(CSiMe3)(PMe3) (4a) was found to undergo a rare, PMe3-promoted exchange with its bis(alkylidene) tautomer (Me3SiCH2)2W(=CHSiMe3)2(PMe3) (4b). Thermodynamic studies of the exchange showed that 4b is favored and gave Keq and the enthalpy and entropy of the equilibrium: DeltaH degrees = -1.8(0.5) kcal/mol and DeltaS degrees = -1.5(1.7) eu. Kinetic studies of the alpha-H migration between 4a and 4b by variable-temperature NMR gave rate constants k1 and k-1 for the reversible reactions and activation enthalpies and entropies: DeltaH1 = 16.2(1.2) kcal/mol and DeltaS1 = -22.3(4.0) eu for the forward reaction (4a --> 4b); DeltaH2 = 18.0(1.3) kcal/mol and DeltaS2 = -20.9(4.3) eu for the reverse reaction (4b --> 4a). Ab initio calculations at the B3LYP level revealed that PMe3 binds with the bis(alkylidene) tautomer relatively more strongly than with the alkylidyne tautomer and thus stabilizes the bis(alkylidene) tautomer.

An equilibrium mixture of alkyl alkylidyne W(CH2SiMe3)3(CSiMe3)(PMe3) (1a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(=CHSiMe3)2(PMe3) (1b) has been found to undergo an alpha-hydrogen abstraction reaction in the presence of PMe3 to form alkyl alkylidene alkylidyne W(CH2SiMe3)(=CHSiMe3)(CSiMe3)(PMe3)2 (2). In the presence of PMe3, the formation of 2 follows first-order kinetics, and the observed rate constant was found to be independent of the concentration of PMe3. The activation parameters for the formation of 2 are Delta H = 28.3(1.7) kcal/mol and Delta S = 3(5) eu. In the presence of PMe2Ph, an equilibrium mixture of W(CH2SiMe3)3(CSiMe3)(PMe2Ph) (3a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(=CHSiMe3)2(PMe2Ph) (3b) was similarly converted to W(CH2SiMe3)(=CHSiMe3)(CSiMe3)(PMe2Ph)2 (4). The observed rate of this reaction was also independent of the concentration of PMe2Ph. These observations suggest a pathway in which the tautomeric mixtures 1a,b and 3a,b undergo rate-determining, alpha-hydrogen abstraction, followed by phosphine coordination, resulting in the formation of the alkyl alkylidene alkylidyne complexes 2 and 4.

Background: DaxibotulinumtoxinA for Injection (DAXI) is a novel botulinum toxin type A formulation in clinical development. A phase 2 dose-ranging study identified an optimal dose and demonstrated efficacy with a median duration of 24 weeks. Methods: In two phase 3, multicenter, randomized, double-blind, placebo-controlled studies (SAKURA 1 and SAKURA 2), subjects with moderate or severe glabellar lines at maximum frown were assigned randomly to receive placebo or 40 U of DAXI. Glabellar lines were evaluated at least every 4 weeks for at least 24 weeks until severity returned to baseline (≤36 weeks). Results: Overall, 609 subjects were enrolled (DAXI, n = 405; placebo, n = 204). DAXI was significantly more effective than placebo in achieving the primary efficacy outcome (≥2-point improvement in glabellar line severity at maximum frown at week 4 according to both investigator and subject ratings): 73.6 percent versus 0.0 percent (SAKURA 1), and 74.0 percent versus 1.0 percent (SAKURA 2) (both p < 0.0001). Composite investigator and subject ratings of maximum frown after DAXI treatment showed that glabellar line severity of none or mild was maintained for a median of 24.0 weeks (SAKURA 1) and 23.9 weeks (SAKURA 2), and glabellar line severity did not return to baseline levels for a median of 27.7 and 26.0 weeks, respectively. DAXI was generally well tolerated, with the most common adverse events related to DAXI treatment being headache (SAKURA 1, 7.0 percent; SAKURA 2, 5.9 percent) and injection-site pain (5.0 percent and 2.4 percent, respectively). Conclusions: Results from both studies were highly consistent. DAXI may offer a prolonged duration of response (median, ≥24 weeks) and is generally well tolerated. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, I.