title: Accumulation and distribution of doxorubicin in tumour spheroids: the influence of acidity and expression of P-glycoprotein.
abstract: OBJECTIVE
  The intra-tumour distribution of anticancer drugs remains an important, but often under-estimated, influence on drug efficacy. Tumour acidity and the presence of efflux pumps were examined for their influence on the distribution of doxorubicin in a solid tumour model.
  
  
  METHODS
  Anticancer drug distribution and overall accumulation was measured in tumour spheroids (TS) of varying sizes. The distribution profiles were examined in normoxic and hypoxic TS, the latter generating metabolic acidosis. Finally, the drug distribution profiles were related to efficacy using radial outgrowth assays.
  
  
  RESULTS
  In large tumour spheroids (TS) (d ~500 μm), intracellular accumulation of doxorubicin was restricted to cells in the outermost layers and failed to accumulate within the viable cells in the 'intermediate' hypoxic zone. A similar profile was obtained for another protonatable amine, 7-AAD. In contrast, the distribution of the non-ionisable drug (at physiological pH) BODIPY-Taxol was uniform throughout the TS. In order to independently model the hypoxic and normoxic zones of TS, we compared drug accumulation in small entirely normoxic TS (d ~200 μm) with equivalent sized ones exposed to hypoxia in an anaerobic chamber. Exposure of TS to hypoxia caused a considerable reduction in the pH of the bathing medium and lower tissue accumulation of doxorubicin. Interstitial acidity reduces the proportion of doxorubicin in the non-ionised form.
  
  
  CONCLUSIONS
  In TS, the accumulation and distribution of doxorubicin was influenced by both the expression of P-glycoprotein and hypoxia-induced acidity. Therefore, optimisation of doxorubicin chemotherapy for hypoxic tumours will require circumvention of both of these crucial pharmacokinetic determinants.
authors.affiliation: Nuffield Department of Clinical Laboratory Sciences, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, UK.
authors.forename: Howard R
authors.initials: HR
authors.lastname: Mellor
authors.forename: Richard
authors.initials: R
authors.lastname: Callaghan
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000143:Acids / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000378:metabolism*; D004317:Doxorubicin / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007150:Immunohistochemistry; D008856:Microscopy, Fluorescence; D009369:Neoplasms / Q000378:metabolism*; D018874:Spheroids, Cellular / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000143:Acids; D000970:Antineoplastic Agents; D004317:Doxorubicin