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Isolated cell systems of human neutrophils (PMNs) and monocyte-derived macrophages (MDMs) were used to compare the destructive potential of these cells during the acute and chronic phases of inflammation, respectively. The contrast in the damage to poly(urethane)s (PUs) was monitored by measuring radiolabel release elicited from a (14)C-polyester-urea-urethane (PEUU) during incubation with both cell types. Human PMN were seeded onto polymer-coated glass slips and both radiolabel release as well as serine protease activity [assayed with N-benzyloxycarbonyl lysine thiobenzyl ester (BLT)] were measured 18 h later. Human monocytes were cultured on polystyrene tissue culture plates for 14 days, trypsinized, and seeded onto the polymer-coated glass slips; then, radiolabel release and esterase activity [assayed with p-nitrophenylbutyrate (PNB)] were measured after 18 h. Coverslips with MDM were also incubated for an additional 2 weeks. At 18 h postincubation with the PEUU, MDM elicited 25 times more radiolabel release per 10(6) cells than PMN at 18 h and continued to increase more than sevenfold over the 18-h value during the subsequent 14-day period. The BLT activity in PMN did not increase significantly during the 18-h incubation period, whereas the PNB activity in MDM increased more than fourfold. The MDM, but not the PMN elicited radiolabel release, was inhibited by the protein synthesis inhibitor cycloheximide, as was the increase in PNB activity. The data provide evidence for a hydrolytic role for MDM and, to a lesser extent PMN, in the biodegradation of implanted materials. The full implication of the release of polymer-derived chemical agents from this hydrolytic cleavage of the implanted biomaterials, on the propagation of the inflammatory response, remains to be elucidated.

Monocytes adherent to implanted biomaterials differentiate into macrophages while synthesizing large amounts of degradative enzymes, including cholesterol esterase (CE), which previously has been shown to degrade poly(urethane)s. Human peripheral blood monocytes were cultured on tissue culture grade polystyrene (PS), and two model poly(urethane)s were synthesized from (1) polycaprolactone (PCL) and (2) polytetramethylene oxide (PTMO), both with 2,4-toluene diisocyanate (TDI) and ethylene diamine (ED). The increase in CE and total protein per cell were measured on days 8 and 28 in culture and normalized to the DNA content per cell. At day 8 there consistently were fewer cells remaining on the PTMO-based polymer than on the PCL-based polymer or the PS (p < 0.05). When comparing day 28 to day 8, there was more CE activity and protein per cell on all materials. However, there was a disproportionate synthesis of CE per mg of total protein on PS and TDI/PCL/ED whereas on PTMO there was not. Significantly, there was more protein and CE per cell on PTMO than on PS or TDI/PCL/ED (p < 0.05). This in vitro model system of the chronic phase of inflammation has shown that it is possible to culture monocytes for a month and assess the material surface itself as a potent activator of the differentiation into macrophages without secondary stimulation. Since CE has been shown to degrade poly(ether and ester)-based poly(urethane)s, the differential production of this enzyme relative to the total protein on different surfaces may impact on the potential long-term biostability of an implanted material.

Biodegradation of poly(urethane)s (PU)s using single enzymes in vitro was assessed by measuring radiolabel release from model poly(ester-urea-urethane) (PESU) and poly(ether-urea-urethane) (PETU) materials synthesized with 14C-labelled monomers. Cholesterol esterase (CE), an enzyme found in monocyte-derived macrophages (MDM), has been reported to cause a significant level of radiolabel release from both of these PUs. Previous work has shown that CE activity could be inhibited by the serine protease/esterase inhibitor, phenylmethylsulfonyl fluoride. Since many serine proteases are present in circulating blood and can be released by cells other than MDM, this study investigated the ability of serine proteases relative to that of CE to cause the degradation of PUs. In addition, the possible role of several oxidative enzymes in the breakdown of PUs was investigated. Proteinase K, chymotrypsin and thrombin, when incubated with PESU, coated on glass slips, caused significant radiolabel release, with proteinase K giving the highest values. However, the highest radiolabel release which proteinase K could elicit was ten times less than CE. Thrombin and then chymotrypsin were progressively worse in their biodegradative activity. Only CE, and not the serine proteases, could elicit a detectable radiolabel release from PETU. Although the release of reactive oxygen species and molecular oxygen occur around an implanted biomaterial, several oxidative systems (peroxidase, xanthine oxidase, catalase), known to produce one or more of these molecular species, were unable to induce radiolabel release from these PUs. The process of biodegradation as assessed by radiolabel release appears to be a specific hydrolytic process, while the role of oxidative enzymes remains less clear.