5-Aminolevulinic acid（ALA）為光敏感物質protoporphyrin Ⅸ（PpⅨ）生合成的前趨物，經由血紅素生成路徑合成PpⅨ，用於光動力學療法。為了探討ALA於皮膚細胞吸收之機轉，本實驗室先前採用各種抑制劑探討輸送載體對皮膚細胞吸收ALA所扮演的角色。以人類基底細胞癌（BCC）預培養胺基酸輸送載體System BETA之受質（b-alanine，GABA），對於ALA的吸收量和PpⅨ的生成量都有顯著的抑制，顯示System BETA參與ALA於BCC的穿透機轉；預培養能量抑制劑（metabolic inhibitor）也可以降低ALA於BCC的吸收，其抑制程度與給予不同濃度之GABA所達到之最大抑制效果近似，推論ALA藉由載體輸送部份需要能量存在。於非致腫瘤性細胞株之人類株化角質細胞（HaCaT），預培養胜肽類輸送載體PEPT受質（Gly-Gly），對ALA吸收的抑制效果相對於其他胺基酸輸送載體受質更為明顯，證實ALA於腫瘤細胞及正常細胞的穿透經由不同的載體調控。
本研究之目的在進一步探討載體於ALA經皮輸送所扮演的角色，選用正常小鼠（ICR）皮膚，以體外及體內模式來評估代謝性抑制劑（NaN3+2-deoxy-D-glucose）和載體抑制劑（b-alanine，Gly-Gly）對經皮滲透速率，表皮、真皮內ALA滯留量和PpⅨ生成量的影響，並比較以不同方式給予抑制劑的效果（預給、後給、或預給＆後給）。
ALA體外滲透試驗證實，四小時試驗之實驗操作方式比六小時較適合探討能量（N-DG）或載體（b-alanine、Gly-Gly）抑制劑之影響。而四小時體外試驗中，預培養抑制劑使ALA之穿皮速率增加，並推論ALA之經皮滲透可能與需要能量的載體運輸有關。正常皮膚體內試驗中，給予抑制劑後可以增加ALA於皮膚內的滯留量，但皮膚內PpⅨ之生成反而增加，結果未與研究假說完全相符。進一步以破壞皮膚障壁之體內模式證實，當抑制劑b-alanine穿透量增加時，ALA滯留量增加及PpⅨ生成減少，其抑制的效果與假說相符，顯示ALA之經皮吸收及PpⅨ的生成，會受載體抑制劑之影響。
綜合以上結果，雖然本研究之數據變異性較大，無法顯示統計上之顯著差異，但亦指出能量（N-DG）或載體（b-alanine、Gly-Gly）抑制劑可能會影響ALA之經皮輸送，在體外模式能增加ALA之穿透速率，於體內模式則是增加ALA之滯留量及PpⅨ生成量。

5-Aminolevulinic acid (ALA), the precursor of haem biosynthesis has been used to induce the endogenous synthesis of the photosensitiser protoporphyrin IX for photodynamic therapy. Previous studies have demonstrated that system BETA transporters may play an important role in the uptake of ALA in skin tumor cells (basal cell carcinoma) and results in selective accumulation of ALA-induced PpIX in tumors. Pre-incubation of system BETA substrates, such as b-alanine and GABA, would inhibit the uptake of ALA into the cells and decrease the formation of PpIX. The inhibition effect of pre-incubating metabolic inhibitors in BCC cells is similar to the maximal inhibition effect of GABA, and confirming the energy dependency of mediated-transport. In human immortalized keratinocytes, the dipeptide Gly-Gly has more significant effect than system BETA does. Evidences show that the cellular uptake of ALA in normal and skin tumor cells can be mediated by different transport system.
The objective of this work was to investigate the role of transporter for the skin transport of ALA. Specifically, metabolic inhibitors (NaN3+ 2-deoxy-D-glucose) and transport inhibitors (b-alanine, Gly-Gly) were applied to examine the skin permeation rate and retention of ALA, and PpIX formation. In in vitro and in vivo mice skin models were used to verify the effect of inhibitors in different conditions, such as pre-load, after-load, and pre & after load.
The results of in vitro permeation studies showed that four hour experiment was more proper for investigation of metabolic inhibitor or transport inhibitors than six-hour experiment, and the transdermal permeation of ALA could be mediated by energy dependent transporter. In vivo studies showed that inhibitors would increase the deposition of ALA and the formation of PpIX in mouse skin. These results didn’t agree with the hypothesis of the study. Loading b-alanine after breaking the permeability barrier by tape-stripping would increase the deposition of ALA and decrease the formation of PpIX, suggesting that transdermal permeation of ALA and formation of PpIX could be mediated by transport inhibitors.
In coclusion, although the significance of the results was compromised by the large variation of the data, the studies suggest that metabolic inhibitor (N-DG) and transport inhibitors (b-alanine, Gly-Gly) could mediate the skin transport of ALA. Inhibitors would increase the permeation rate of ALA in in vitro studies, while in in vivo models, they would increase the deposition of ALA and decrease the formation of PpIX.

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