The wide range of tumour pH values that have been determined in human tumours is shown
in Fig. 4. It can be seen that tumour pH values may be very low, or may fall in the
same range as the values found in normal tissues. This means that pH-mediated modification
of therapeutic effectiveness will be patient specific, rather than a general phenomenon.
That the pH of the cellular environment might influence the effectiveness of various
therapeutic agents is not a new idea. The data published in this field to date concerning
such effects have been discussed extensively and are summarized in Table IV. Here
we can see that low pH leads to decreased cell survival following treatment with hyperthermia,
radiotherapy combined with hyperthermia, radiosensitizers and various chemotherapeutic
agents. Conversely, low pH affords some protection against radiation and some drugs.
Most of these data were, of necessity, derived from in vitro studies. In vivo studies
are in most cases not feasible due to the difficulty of isolating the effect of one
selected factor. Low tumour pH is, in vivo, generally assumed to be closely interlinked
with tissue hypoxia and low blood-flow levels, each of which may individually influence
the experimental outcome. Moreover, most of the aforementioned in vitro studies were
conducted under well-oxygenated conditions. As previously mentioned, euoxic cells
can, under certain conditions, maintain a pH gradient over the cell membrane. This
collapses with the onset of hypoxia, leading to intracellular acidification. Low oxygen
levels have been shown to be characteristic of many tumours. Within these limitations
it is thus evident that tumour pH values could have far-reaching consequences for
therapy. If the in vitro findings should prove to be relevant to the clinical situation
various applications are possible. Pre-selection of patients less likely to respond
to certain (toxic) chemotherapeutic agents, or conversely selection of agents that
are more likely to be effective in the pH range of the tumour to be treated are two
examples. Alternatively, the exploitation of low tumour pH values is a possibility.
Agents that form or release toxic derivatives in areas of low pH, e.g., pH-sensitive
liposomes, will work selectively in such areas. Tumour selective therapy may also
be possible in patients with higher tumour pH values if the tumour pH can be lowered.
This has been achieved experimentally by the administration of hyperthermia at temperatures
above 42 degrees C, or by the administration of glucose.(ABSTRACT TRUNCATED AT 400
WORDS)