A. Faucitano

Gamma irradiation effects on poly(dl-lactictide-co-glycolide) microspheres

Gamma radiation treatment plays an increasingly important role in the sterilization/sanitization of pharmaceutical products. However, irradiation may affect the stability of the product and thus its safety of use. We investigated the influence of ionizing radiation on modified release microparticulate drug delivery systems made of two types of polylactide-co-glycolide copolymers (PLG): RG 503 and RG 503H; these polymers have identical molecular weights but different chemical structures. The effect of gamma radiation on polymer stability of the raw polymers (P) and related microspheres (Ms) was evaluated. Samples were irradiated at different irradiation doses (5, 15 and 25 kGy) using 60Co as radiation source. The microspheres were prepared using the spray drying technique. Degradation of PLG and related microspheres was evaluated during six months in terms of average molecular weight (Mw) loss by gel permeation chromatography (GPC) and variation in glass transition temperature (Tg) using differential calorimetry (DSC). The presence of free radicals in the product was tested by electron paramagnetic resonance (EPR). Both P and Ms showed a trend in decreasing their Mw at time 0 as a function of irradiation dose. For RG503 the decay in Mw is always negligible for doses below 15 kGy while it is about 10% for 25 kGy. After 150 days Mw decay was 25% in the microspheres and 20% in the raw polymer. It was not possible to evaluate the radiation effect, at different storage times, for RG503H because this polymer resulted to be unstable even in the regular storage conditions without being irradiated. The concentration of radiation-induced free radicals was higher in RG 503H (both P and Ms) and they were more stable than the free radicals species observed in the case of polymer RG 503. Alterations and/or production of new radicals were observed on exposure of RG 503H microspheres to the light. Radiolytic degradation of RG 503 under vacuum is characterized by a prevalence of the chain scission events leading to a decrease of Mw. Some crosslinking can occur mainly in the post irradiation stage through the decay and coupling of the hydrogen abstraction radicals. A hydroperoxydative cycle, whose mechanism is suggested, is generated in the presence of oxygen.

Gamma irradiation effects on stability of poly(lactide-co-glycolide) microspheres containing clonazepam.

This work was aimed at evaluating the effects of gamma irradiation on the stability of microspheres made of a poly(lactide-co-glycolide) copolymer (PLGA) and loaded with 15% w/w of clonazepam (CLO). The influence of CLO on PLGA radiolysis mechanisms and the identification of possible irradiation markers were also investigated. Microspheres were prepared by means of a spray-drying method. gamma Irradiation was carried out either under vacuum or in air, at a dose of 25 kGy, by using a 60Co source. The stability of CLO loaded microspheres was evaluated over a 6-month period on the basis of drug content and dissolution profile. Radiolysis mechanisms were investigated by using electronic paramagnetic resonance (EPR) analysis. The microspheres irradiated under vacuum were stable over the considered period of time. After irradiation in air, CLO release rate increased by approximately 10%, and did not change further in the following period of storage. The EPR analysis showed some radicals arising from both the polymeric matrix and the active ingredient. Polymer/CLO spin transfer reactions suggest that CLO had a radio-stabilising effect on the polymeric matrix. In the loaded microspheres, the intensity in time of the CLO radical signal is sufficient for its possible use as irradiation marker.

Gamma irradiation effects and EPR investigation on poly(lactide-co-glycolide) microspheres containing bupivacaine.

The effects of gamma radiation on the stability of microspheres made of a polylactide-co-glycolide 50:50 copolymer (PLGA) and loaded with 40% bupivacaine (BU) were studied. The radiolysis mechanisms of BU and BU-loaded microspheres were investigated by using electronic paramagnetic resonance (EPR) analysis. Microspheres were prepared by means of a spray drying method. Gamma Irradiation was carried out in the open, at the dose of 25 kGy, by using a 60Co source. The stability of BU-loaded microspheres was evaluated over a 1-year period on the basis of drug content and dissolution profile. Non-irradiated microspheres were stable over the whole period under consideration. Immediately after irradiation the amount of BU released after 24 h from irradiated microspheres increased from 17 to 25%; in the following 3 months of storage it increased to about 35%, and then it kept constant for 1 year. Radicals generated by BU irradiation were identified by EPR analysis; the sensitivity to gamma radiation of BU was about four times lower than that of PLGA. Furthermore, the EPR spectra of loaded microspheres showed that the relative abundance of BU radicals plus PLGA radicals was proportionate to the electronic fractions of the components; this implies that no spin transfer BU/PLGA had occurred during gamma irradiation.

γ-irradiation of PEGd,lPLA and PEG-PLGA Multiblock Copolymers: II. Effect of Oxygen and EPR Investigation

The purpose of this research was to evaluate how the presence of oxygen can affect irradiation-induced degradation reactions of PEGd,lPLA and PEG-PLGA multiblock copolymers submitted to gamma irradiation and to investigate the radiolytic behavior of the polymers. PEGd,lPLA, PEG-PLGA, PLA, and PLGA were irradiated by using a 60Co irradiation source in air and under vacuum at 25 kGy total dose. Mw and Mn were evaluated by gel permeation chromatography. The stability study was carried out on three samples sets: (a) polymer samples irradiated and stored in air, (b) polymer samples irradiated and stored under vacuum, and (c) polymer samples irradiated under vacuum and stored in air. The thermal and radiolytic behavior was investigated by differential scanning calorimetry and electron paramagnetic resonance (EPR), respectively. Samples irradiated in air showed remarkable Mw and Mn reduction and Tg value reduction due to radiation-induced chain scission reactions. Higher stability was observed for samples irradiated and stored under vacuum. EPR spectra showed that the presence of PEG units in multiblock copolymer chains leads to: (a) decrease of the radiolytic yield of radicals and (b) decrease of the radical trapping efficiency and faster radical decay rates. It can be concluded that the presence of oxygen during the irradiation process and the storage phase significantly increases the entity of irradiation-induced damage.

Quenching of singlet oxygen by hindered amine light stabilisers. A flash photolytic study

Rate constants for the quenching of singlet oxygen (1O2, 1δg) for a series of piperidines, piperidine-N-oxyl free radicals and some commercially used hindered amine light stabilisers (HALS) have been measured by a laser flash photolysis method. Quenching rate constants are in the order: piperidine-N-oxyl free radicals ≤ secondary piperidines < tertiary piperidines. For some commercial HALS, 1O2 quenching rate constants and the light protective effect towards polypropylene photo-oxidation have been compared. No correlation has been found between the stabilizing action and the quenching efficiency towards 1O2. The data obtained point to little contribution of singlet oxygen to the key steps of polyolefin photo-oxidation.

The radiation chemistry of perfluorinated ethers

Abstract The investigation on the radiation chemistry of the perfluoropolyethers carried on through the analysis of the free radicals intermediates, of the structural changes in the oligomers chains and with the support of M.O. calculations has afforded information on the major features of the mechanism of radiolysis. The main radiolytic process is chain degradation by CC and CO bond rupture. Evidence of a minor participation of CF bond scissions have been obtained in the case of poly(perfluorotrimethylene oxide) PF3MO and poly(perfluoroethylene-perfluoromethylene oxide) PFEMO. The nature of the radicals intermediates and the structure of the novel units produced in the oligomer chains fit a radiolysis mechanism where both the ionic and excitation paths seem to be operative giving rise to complex sequence of radical reactions. The structure and possible reaction modes of the molecular cation and anion radicals are discussed on the basis of PM3 semi-empirical M.O. calculations.

ESR study of the direct radiolysis of DNA, DNA-histones and DNA-intercalators complexes

Abstract The nature of the radicals contributing to the room temperature spectrum of irradiated “dry” DNA, with special reference to the central structure, is discussed, and the thesis of their ionic origin tested by irradiation experiments with intercalators. The mechanism of spin transfer protein→DNA has been investigated through a comparative ESR study on the DNA-histones complex, the structureless random molecular mixture of the DNA-histones and the neat components. The yield of spin transfer is enhanced in the random mixture, presumably because of the greater efficiency of molecular contacts. Evidence of the scavenging of electrons by the thymine and cytosine bases, as a key mechanism for the spin transfer, has been obtained.

Ionic and excited intermediates in pulse-irradiated polypropylene doped with aromatics

A pulse radiolysis study of isotactic polypropylene (PP) film has been carried out with the main aims of investigating charge trapping in an undoped system and solute radical ion generation in an pyrene (Py) doped matrix. In PP, pulse radiolysis gives electron–positive hole pairs. The electron can be stabilized in the undoped system as a trapped electron, e. The transient absorption spectrum of e in the near-IR (up to 1800 nm) was observed in the temperature range 30–100 K. This IR absorption was not detected in the case of oxidized PP. In such a matrix electrons can be scavenged by oxidation products generating respective radical-anions (absorption in the UV RANGE, λ Tg and T < Tg, respectively. Some preliminary results concerning the ionic processes in PP containing two solutes (Py, 3,3′-dimethyldiphenyl) were presented. The mechanism of ionic recombination in PP will be proposed and discussed.

Free-radical and ion chemistry of fluoro derivatives : the mechanism of γ-radiolysis of perfluoropolypropene and perfluoropolytrimethylene oxides

Abstract The radiation chemistry of perfluoropolypropene and perfluoropolytrimethylene oxides has been investigated by ESR spectroscopy of the intermediate free radicals and by NMR spectroscopic detection of the major changes occurring in the structure of the oligomers. The major radiolytic products have been related to CC and CO bond scissions and to a minor participation of CF bond cleavage. The results have been rationalized in terms of ionic and free-radical reactions stemming from the formation of primary ether cation-radicals.

Esr study of intermediate radicals in UV and gamma irradiated perfluoropolyethers and their related polyperoxidic precursors1

Abstract This note is concerned with an esr study of intermediate radicals formed by UV and gamma irradiation of the perfluoropolyethers A,D and their peroxidic precursors B,C obtained by perfluoroolefines photooxidation 2 CF 3 (O-CF 2 CF(CF 3 ) ) n  (OCF 2 ) m  OCF 3 (A) Y(OCF 2 CF(CF 3 ) ) n  (OCF 2 ) m  (O) p  Y′ (B) Z(OCF 2 CF 2 ) n  (OCF 2 ) m  (O) p  Z′ (C) CF 3 (OCF 2 CF 2 ) n  (OCF 2 ) m  OCF 3 (D) Y = −CF 3 , −COF; Y′ =  −OCF 3 , −OCOF; Z = −CF 3 , CF 2 COF; Z′ = −OCF 3 ,  OCF 2 COF The UV irradiation at 4.2 K of B leads to the formation of the radicals CF 3 ·, −OCF 2 · and −O(CF 3 )CF·. The same species are detected folling UV irradiation at higher temperatures with the samples in the liquid state. Above 400 K, the esr spectrum of −OCF 2 · and, partially, of −O(CF 3 )CF·, are replaced by that of −0CF 2 (CF 3 )CF·. The UV irradiation of C always leads to the exclusive formation of −OCF 2 ·. These observations are consistent with the following reaction schemes: The analysis of the esr spectra obtained following gamma irradiation at 77 K of the peroxides B,C point to the major process for neutral radical production being based on the peroxidic OO bond rupture followed by the β scission of secondary fluoroalkoxyls. In the case of the neutral polyethers A,D the rupture of the ether CO bonds seems to be of prominent importance. In all cases, strong singlets centred at g∼ 2.003 are observed; these signals are tentatively assigned to radical anions or trapped electrons on the basis of the results of irradiation experiments with charge scavengers and with samples of B prepared by 17 O 2 photooxidation of perfluoropropene. Informations regarding the electronic structures and the geometries of the alkoxyfluoralkyl radicals identified have been obtained from the anisotropies of α −F tensors and from INDO SCF MO calculations. The computed energy minima and the best agreement between computed and experimental esr data are obtained by assuming pyramidal configurations of the radical centres with degrees of bending equal to ∼ 11° and ∼ 17° for −OCF 2 (CF 3 )CF· and −OCF 2 · respectively. Such large deviations from the planarity are considered diagnostic of a destabilizing π conjugative interaction with the α O atoms. The esr investigation has been extended also to the peroxyl radicals RO 2 · obtained by reaching −OCF 2 (CF 3 )CF· and −OCF 2 · with 17 O 2 . The species R 16 O 17 O and R 17 O 17 O have been identified and characterized. A summary of the experimental and calculated (INDO method, between brackets) h.f. parameters of the radicals identified is reported in the following table: