Synopsis - Chlorophyll Biosynthesis and Technological Applications - page 57

The tetrapyrrole-dependent connotation of this herbicidal system is meant to
differentiate between this class of photodynamic herbicides from other light
activated herbicides such as paraquat which are not dependent on tetrapyrrole
metabolism for herbicidal activity. During the past 20 years, the scope of TDPH
research has expanded considerably, as some established herbicides which act via
the TDPH phenomenon have been discovered.
The discovery of photodynamic herbicides and their development will be
discussed at length in Chap. 17 .
2.10.6 Discovery of Porphyric Insecticides
The discovery of porphyric insecticides (Rebeiz et al. 1988a ) was built upon the
discovery and development of photodynamic herbicides (see above). In this case
however the tetrapyrrole which accumulated upon incubation of insects with ALA
and modulators was identified as Proto, the precursor of heme in insects and
animals, and of Chl in plants. Proto in turn generates damaging singlet oxygen
upon light exposure as was observed in plants. The discovery and development of
porphyric insecticides will be described in Chap. 18 .
2.10.7 Discovery of Photodynamic Cancericides
Following in the footsteps of the photodynamic herbicide and porphyric insecticide
technologies, the porphyrin-inducing properties of delta-aminolevulinic acid (ALA)
have been adapted for the photodynamic destruction of cancer cells (Kennedy
et al. 1990 ). Topical administration ALA to various skin lesions has, in particular,
been very successful in clinical trials. For instance Kennedy et al. ( 1990 )treated
successfully basal cell carcinomas, superficial squamous cell carcinomas, and actinic
keratoses, with a response rate of 90% for basal cell carcinomas. Overall, topical, oral,
or systemic administration of ALA and subsequent photodynamic therapy has
been successful in a variety of tumor models, including amelanotic melanomas
(Abels et al. 1994 ), pancreatic cancer and colon tumors, and breast cancer (Rebeiz
et al. 1996 ). Description of the development of ALA-dependent photodynamic
cancericides will be discussed in Chap. 19 .
2.10.8 Chloroplast Bioengineering
The notion of chloroplast bioengineering stems from the observation (a) that
conventional agriculture is one of the few human activities that have not undergone
a revolution to join other activities such as defying gravity by flying and landing on