Reduction of an enzyme required for the lysosomal degradation of glycosaminoglycan (gag) chains will result in a mucopolysaccharidosis (MPS) disorder. Substrate deprivation therapy (SDT), a new therapy option for MPS, aims to reduce the synthesis of gag chains, the natural substrate for the deficient enzyme. Reduced substrate levels would balance the reduced level of enzyme in patient cells resulting in normalised gag turnover. Rhodamine B, a non-specific inhibitor, reduced gag synthesis in a range of normal and MPS cells and also decreased lysosomal storage of gag in MPS VI (72%) and MPS IIIA (60%) cells. This positive response in vitro was extended to an in vivo therapy trial in the MPS IIIA mouse. Bodyweight gain of male MPS IIIA mice treated with 1 mg/kg rhodamine B was reduced compared to untreated MPS IIIA mice and was indistinguishable to that of normal mice. Liver size, total gag content and lysosomal gag was reduced in treated MPS IIIA animals as was urinary gag excretion. The alteration in MPS IIIA clinical pathology by rhodamine B combined with the observation that treatment had no effect on the health of normal animals demonstrates the potential for this type of therapy for MPS disorders. The water cross maze was found to be the only learning and memory test capable of detecting differences in learning behaviour in MPS IIIA and normal untreated mice. MPS IIIA mice treated with SDT rhodamine B showed an improved outcome with better learning capabilities than MPS IIIA untreated mice observed using this test. This means that rhodamine B is likely to cross the blood-brain barrier. These results are the first evidence of a positive response by the CNS to a systemic therapy for MPS IIIA. Rhodamine B administration over 4 generations did not produce any deleterious side effects in MPS IIIA. In utero therapy over four generations did not cause a reduction in litter size or bodyweight profile demonstrating that reduction of gag over a combined timeframe of two years was safe. A higher dose of 5 mg/kg rhodamine B did not produce any additional benefits on MPS IIIA pathology and no signs of hepatoxicity were noted. Rhodamine B proved to be a general inhibitor of gag synthesis and had a positive outcome on a number of clinical parameters in MPS IIIA mice. SDT in MPS IIIA mice improved learning capabilities as detected by the water cross maze which has not been previously reported. This provides evidence that small molecules such as rhodamine B, that are able to cross the blood-brain barrier, can have some effect on neurological pathology. This proof of principle study showed that SDT can be used to have a positive outcome on MPS pathology. Additional inhibitors of gag synthesis can also be investigated before this type of therapy can be translated into clinical use in MPS patients. Although it may be feasible to use rhodamine B as a SDT agent in vivo, other inhibitors may be more practical. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1287050 / Thesis (PhD) -- School of Paediatrics and Reproductive Health, 2007.